@2

It's not really the sort of thing you see every day is it?" Kristin agreed. "Which is good because it's so much hotter when it has to be a secret."

"Yeah, what would be the fun having sex with your brother if everyone was doing it?" I added with only a small amount of sarcasm.

Kristin rolled her eyes but leaned in to suck the head of my cock into her mouth instead of responding. Between her hand and having someone watching us I'd gotten hard fairly quickly and it felt good to have my sister start getting serious about it instead of just playing with me.

The fact that not too long ago the girls' positions had been reversed wasn't lost on me, Kristin having seen Shannon giving me a blowjob in my room. I hoped there wouldn't be too much awkwardness arising from that, but Shannon certainly didn't seem too put out. Especially since she was the one who said she wanted to see it.

When it came down to it, I couldn't figure out precisely what Shannon was thinking. Her face wasn't giving me enough information to decide whether she was merely curious or whether she was actually enjoying what she saw on some perverse level. It was a very typically guy thing, but I couldn't help wondering what might happen if she decided she really liked watching us. There was some fantasy material there if I chose to take it.

"Okay, you're right. This is pretty hot," Shannon said eventually.

Kristin had my cock filling her mouth, but still managed a muffled sound of agreement. As my attention was drawn back to my sister I noticed that her hand was down the front of her pants, somehow I'd failed to catch that she was getting herself off at the same time as me.

Shannon still wasn't openly masturbating or anything overt, but when I looked for it I saw that she was fidgeting in her chair and squeezing her legs together every now and then. She was definitely getting horny watching us even if she wasn't comfortable enough to do something about it.

I returned my attention to Kristin who continued to suck my cock and bring me closer to orgasm while simultaneously getting herself off. Since neither of her hands were free I brushed some of her hair back out of her face where it had fallen, tucking it behind her ear. She looked up at me and deliberately took me deeper on the next couple down strokes, letting my cock go far enough for me to feel the back of her throat before pulling away again.

I heard a small, barely noticeable sound from Shannon's direction and when I looked over a moment later her hand was between her legs, clamped down firmly over her crotch as she squeezed her thighs together. It looked like she was putting some pressure on her clit, possibly just enough stimulation to keep from going crazy. I thought about telling her it was okay if she wanted to masturbate, but she seemed a little embarrassed just at having me notice her and I didn't want to add to that.

Before long my sister's efforts paid off and I felt the familiar sensation of impending orgasm.

"Getting close," I said softly.

Kristin gave me an indecipherable grunt of acknowledgement the best she could do with her mouth still full, and pumped me a little harder. Just before I came she pulled her mouth off me and tilted her head back with her eyes closed. As my semen sprayed out it landed mostly on her waiting face, some of it even inside her mouth, with the last shot falling short and midway down her stomach on the front of her shirt. I was kind of surprised that she would let me cum on her like that with no warning, until I remembered that she was putting on a show for Shannon. In that context it made perfect sense.

My sister stayed exactly where she was for another minute or two with her hand frantically rubbing her pussy and her face partly covered in my cum. Finally she gave a contented sigh and reopened her eyes.

"So was that good enough for you?" Kristin asked, turning to Shannon who had a look of even stronger disbelief than before.

"Uh, yeah," Shannon said. "More than I was expecting actually."

Kristin grinned and stood up, taking stock of her appearance as she did.

"Guess I probably can't wander around like this," she said with just a hint of regret in her voice. "Be back in a minute."

Without making even a token preliminary attempt to wipe herself off she stepped out into the hallway in the direction of the bathroom and left Shannon and me alone in the room. There was a moment of awkward silence before I decided talking might be better than sitting around worrying about what Shannon was thinking.

"So at least you're not freaking out," I said. "That's a good sign right?"

"I suppose," Shannon replied slowly. Her gaze drifted away from the doorway where Kristin had left back to me. "I didn't think I would anyway, it's not like this came out of nowhere. I've had time to think about it. The only thing is I wasn't really sure how far you two had gone."

"Guess you have a better idea now," I said.

"No kidding. Can I ask you something though Kyle?"

"Sure."

"Would you mind if I watched you guys more, kinda like what we just did. Not necessarily all the time or anything, but maybe-"

"It's fine with me," I interrupted, waving my hand casually. "I mean it's not like we've never, you know, done anything."

Granted the total of what Shannon and me had done together was that one blowjob she'd given me, but it was enough to make me a little more comfortable around her. Comfortable enough that I didn't mind if she wanted to continue being a spectator of sorts. Especially since there was a good chance if we did this more she'd give in eventually and masturbate for real. I couldn't help having a certain amount of interest in that possibility.

Before Shannon could respond Kristin walked back in and went straight for her dresser.

"How come you didn't tell me you got cum all over my shirt?" she demanded, giving me a mock glare as she passed me.

"First off I thought you would have noticed," I said. "Second, it really wasn't that much."

Kristin stuck her tongue out at me, then pulled her shirt up over her head without a hint of hesitation despite both Shannon and me still being there. Her breasts weren't anything I hadn't seen of course, and for all I knew Shannon might have seen them plenty of times as well since I understood girls were more open about that kind of thing than guys. All it really did was remind me just how short a time it had been since being half, or fully, naked around each other wouldn't have happened. Now it was quite literally an everyday occurrence.

"So what were you guys talking about?" Kristin asked as she bounced down onto the bed beside me after getting a new shirt.

"Just stuff," I said.

"Cool. So you decide if you're going to help us yet?" Kristin asked Shannon.

Shannon's eyes flicked toward me for a split second, then back to Kristin.

"Yeah, I'm in," she said.

"Good, then everything's going perfectly so far," Kristin said. "Tomorrow I'm gonna have to go talk to Miss Reid and get things arranged with her, then we'll be all set."

****

The next day Kristin decided to wait until the end of the last period to approach Miss Reid, which I agreed with. It made sense to minimize the chance of getting interrupted in the middle of their conversation, and it would make it easier to locate the teacher since she'd almost certainly be in her class packing up for the day and getting ready to go home.

My sister found me just after the final bell while most of the students were surging toward the exits and dragged me into a semi-private alcove. I looked around but no one gave us so much as a second glance as they went by.

"You talk to her yet?" I asked, keeping my question deliberately vague on the off-chance someone actually could overhear us.

"Nope, just about to," Kristin said. "Slight change of plans though, I need you and Shannon to give me about ten minutes then follow me in."

I arched an eyebrow. "Oh yeah, why's that? We're not kidnapping her now are we?"

"Don't be silly. I'm going to try giving her a plausible explanation for our request is all, I would have thought you'd be happy to hear that," she said, poking me sharply in the chest.

"Yeah yeah, alright. I assume you told Shannon already."

"Yep, at lunch. She'll meet you... I don't know, somewhere. Anyway, I gotta go. See you in ten."

Kristin leaned over and kissed me so quickly I didn't have a chance to register the movement before she was already finished and walking away.

"You can't..." I began automatically, then sighed and gave up. Kristin had known exactly what she was doing and telling her off wasn't going to help anything. All I'd do was draw unwanted attention if I tried.

With nothing better to do I followed in generally the same direction my sister had gone, although at a much slower pace. I didn't really have anything to occupy myself for ten minutes other than just standing around keeping an eye on one of the clocks scattered through the hallways so I'd know when it was time. I could have gone to try and find Shannon I suppose, but I had no idea where she would be so it seemed kind of pointless.

I'd only just made it to the outside of Miss Reid's class and leaned against the wall to wait when Shannon found me instead.

"Hey Kyle," she called.

I looked back down the hall where I'd come from as she approached. I hadn't noticed her behind me, but she might well have turned off from somewhere else. Or she'd been following me to see if she could sneak up on me, although that was more my sister's thing.

"Hey," I said back.

Shannon stopped next to me and dropped her bag onto the floor as though it was too heavy to bother carrying it while we weren't going anywhere. Knowing how many textbooks some students carried around with them that might actually be the case.

"Did she tell you what exactly she needs us for?" I asked.

"Nah, not anything specific. We might just be to back up her story, whatever it is."

"Which would work better if knew ahead of time what the story was."

Shannon shrugged. "Not much we can do about that now."

"I know."

I felt a little better having Shannon there waiting with me. It meant I didn't look completely suspicious hanging around by myself, plus it wasn't a terrible thing to be seen with girls who weren't my sister just in case there was someone who might connect the dots on our relationship.

"So it's kinda strange, but I'm actually getting into this a little," Shannon said. "The whole thing with you and Kris."

"You mean watching us?"

"Yeah, except more than that too. Like... okay I don't know if I should really tell you this but last night I went online and, well, I found some videos of brothers and sisters doing stuff. Porn videos I mean." Her face started to get red as she spoke and she avoided meeting my eyes directly, but she kept going. "Obviously they weren't all real or anything, but there were a couple of them that could have been."

"Wow," I said for lack of anything better. "We made more of an impression than I thought."

"Yep, you've corrupted me for good," she said. "Or maybe it's partly the novelty of it, I don't know. I'm definitely looking forward to next time though," she finished a little shyly.

"The real thing's better than porn huh?"

Shannon nodded emphatically, which for some reason made me feel good.

"You think we waited long enough yet?" she asked, gesturing toward the classroom we were standing outside.

"I don't know, close enough I guess."

I opened the door and let Shannon go first before pulling it shut behind me just in case. Kristin and Miss Reid were indeed inside, my sister sitting casually on the teacher's desk with her legs dangling over the edge as the two of them talked. Their conversation had temporarily stopped as we interrupted, but Kristin grinned as she saw us come in.

"You remember Kyle don't you Miss?" Kristin said. "And I'm not sure if you've ever met Shannon...."

"Um, no I don't think so," Miss Reid said a little hesitantly.

"Well anyway they're the ones who are going to be practicing with me," Kristin said. "I'm sure you can trust Kyle can't you? He's always been quite respectable as far as students go."

Miss Reid didn't comment directly on that statement, but her face showed a hint of agreeing with my sister's assessment of me. I didn't know whether to be proud about that, or annoyed. Maybe both.

"You're Kristin's brother aren't you?" Miss Reid asked me. "And you ended up in the same drama class. It's funny I never thought you'd be interested in that."

Neither did I. Nevertheless that was apparently the story we were going with so I was better off not correcting her.

"I think he misses me too much when we're apart, he likes to get in the same classes as me when he can," Kristin said before I could speak for myself.

"Or maybe someone just likes to mess with my course selection list," I replied, choosing something halfway plausible, if untrue, that my sister might have done.

Kristin stuck her tongue out at me, then quickly assumed an innocent expression as Miss Reid turned to her. The teacher looked like she was going to say something, but then shook her head in exasperation. She must have been used to my sister's antics.

"You only need the class for one period right?" Miss Reid asked.

"Yes, just enough so we can practice our skit we need to do," Kristin said. "Doofus over there doesn't want to do it where people can watch for some reason."

I rolled my eyes at the name calling and gave my sister a disbelieving look like she couldn't possibly think I wasn't going to remember that later.

"Alright, well you can have it fourth period on Friday if you really want it," Miss Reid said. "I expect to find everything the way I left it though, if you have to move desks around at least put them back afterward."

I got the impression she was trying to get rid of us at this point, and now that Kristin had what she wanted I had no doubt we'd be leaving.

"Absolutely, you won't even know we were there," Kristin assured her, hopping off the desk where she'd been sitting.

My sister grabbed my arm and dragged me out the door leaving Shannon to follow behind us looking quite amused at everything.

"Doofus huh?" I said as soon as we were out of earshot.

"Just being me," Kristin said with a dismissive wave of her hand. "Don't want to arouse suspicion do we?"

"Yeah, I'm sure that's all it was."

The three of us appeared to be alone, at least temporarily, so I decided I could risk getting a little payback. Kristin was walking right beside me so I moved my hand to her butt and gave it a solid pinch. My sister squealed and jumped at the sudden contact, then whirled on me. I had already backed away and was doing my best not to burst out laughing at her reaction, but was unable to restrain myself completely.

After a moment Kristin gave in and giggled, knowing full well that she deserved it. Shannon just smiled and shook her head at us.

"Well if you two don't need me anymore I have to get going," she said.

"Okay. Remember, Friday fourth period," Kristin said.

"I'll be there."

Shannon gave me a small wave, then headed off leaving us by ourselves.

"Guess we should probably head home too," I said.

"Uh uh, not right away," Kristin said. "First we need to go shopping."

"Shopping? Why?"

"'Cause I need a new skirt. I don't have anything that'll work for Friday, not properly." She leaned in close to whisper conspiratorially. "And you'd rather be able to bend me over the desk, flip up my skirt and fuck me than dick around with getting my pants off wouldn't you?"

"Dammit Kris," I said.

The mental image I got off that was hotter than what was helpful at the moment. I didn't need to be walking around with an erection right now even if most people had left the area already.

"Why do you think I insisted on us bringing the car this morning anyway? It's not like we needed it to go straight to school and back."

"I did wonder about that," I said. "Honestly I assumed you just wanted stop and mess around before we got home."

"Well I wouldn't count that out either." Kristin grinned wickedly. "After all I might need some new panties while we're there, and you'd get to help me pick them out. We never did replace that pair you ripped a while back."

"If I say I'll go will you please stop saying sexy things? At least until we're in the car."

"Hm, I guess that's a good deal. Didn't plan on getting you too worked up yet anyway. Save that for later."

In immediate defiance of her previous statement Kristin turned and walked ahead of me to where I'd parked the car that morning, swinging her hips in exaggerated motions to keep my attention. I thought about telling her to cut it out, but knew even as the thought came to me that I wouldn't be fooling anyone; I loved watching my sister's ass and she knew it as well as I did. At least once we were around people again in whatever clothing store she chose she'd have to behave herself a little. I hoped.

****

The place Kristin decided on turned out not to be all that busy when we got there, but with enough customers wandering around that she couldn't tease me too badly. I kept an eye out for anyone I recognized because as long we remained anonymous I knew could pass easily enough for boyfriend and girlfriend rather than brother and sister.

As interesting as Kristin had promised the shopping trip would be, it didn't quite turn out that way. Searching through an array of skirts for just the right one was not really my idea of a good time, and picking out panties wasn't enough to make up for that. As much as I liked choosing underwear for my sister, I wasn't actually getting to see what she looked like in them which cut down on the appeal somewhat.

"What do you think of this one?" Kristin asked, holding up yet another skirt for my opinion.

"Isn't that the same as the last one?" I said.

"No, it's a little longer and the materials different. I think I like it better, feels nicer."

"Then go buy it so we can be done."

"Aw, you getting tired of this already? Just hang on a few more minutes and I promise it'll get better. I think I'm gonna go try these on."

She had the two not-the-same skirts as well as two pairs of panties that we'd decided on. I figured as long as she wanted my input I might as well see how small I could go before she'd refuse. As it turned out she didn't seem to have a problem trying out some that didn't seem like they'd really cover anything properly. That at least had managed to stir up my imagination a little.

There were some semi-secluded changing rooms toward the back of the store and Kristin entered one while I remained outside. I was sure boyfriends getting dragged along happened regularly enough that I wouldn't stick out too much standing there on my own in the clearly meant-for-girls store, but that didn't mean I was going to like it. I felt quite awkward and not sure where to look, as well as having the small but persistent fear that someone would recognize me. I couldn't even wander around pretending I was browsing because that really would draw attention.

Finally, after more time than I felt it should have taken just to try on a couple things, Kristin opened the door to the dressing room and poked her head out.

"Come in here and tell me what you think," she said.

"I'm pretty sure I'm not supposed to be in there with you," I said.

"You big baby, no one's even going to care. Either get in here or I'll come out there and I don't think you want that."

"Fine."

If we got kicked out I had every intention of telling her "I told you so", but I hoped she was right that it wasn't that big a deal. Or at least that no one noticed.

"So I'm kinda liking this one, you think it's okay?" Kristin asked as I closed the door behind me.There wasn't a lot of space with both of us inside, but it wasn't too cramped either. Kristin was standing in front of the mirror examining the skirt she had put on critically. I thought she looked hot in it, but didn't have much of an opinion otherwise. Plus I thought she looked hot in most anything.

"It's fine," I said.

"You don't think it's too long? I didn't want to go too short and get in trouble, just keep it at the right length."

The bottom of the skirt came down to maybe a couple inches above her knees and I knew that given our school policy she wasn't going to get away with much more than that.

"You look great," I insisted.

"Wouldn't matter what I had on you'd still say that, don't know why I bother sometimes," she said.

"Because you like hearing me tell you how good you look probably."

"Oh yeah, that's it." She giggled. "I guess if I really wanted compliments I'd go a step further."

In one deliberate, smooth motion Kristin pulled the skirt down to her ankles and stepped out of it. Underneath she was wearing one of the pairs of panties we'd picked out, and it was just as useless at properly covering her as I'd hoped. From behind the panties did little more than draw a nice dividing line down the middle of her ass, and when she turned around I saw that they weren't doing much better in front. Her pussy was barely covered and if she had any hair down there it would be clearly showing around the edges of the material.

"I'm guessing you like them," she said.

"Uh, yeah."

"Even though they're really, really small in the back?"

As if that was a downside for me.

Kristin took my hand and pulled it around to her lower back, then down to the top of her butt.

"I bet they're just gonna ride up on me and disappear, hardly even worth wearing them," she said.

I ran my finger down the centre of her ass, pressing the panties between her butt cheeks.

"You might be right," I said.

"Uh huh, but I know you don't really care do you? You'd like it if I didn't wear any underwear."

"I might."

As I continued running my finger up and down the line of her panties, pushing them as deep as I could, I encountered something I didn't expect. There was something there, something that seemed very deliberately placed, and I could tell that Kristin had been waiting for me to find it.

"Is that..." I started.

"A butt plug, yeah," she confirmed. "I got it after we talked about you fucking me back there. Actually I got two, the other one's a little bigger for when I get used to this one. I think I'm almost ready 'cause I sometimes forget I even have this one still in. Told you I'd work on it."

"And you were wearing it all day?"

"Yep. Had to get off in the bathroom at lunch when I got too horny, but it's totally worth it. Feels good when I play with it too."

A shiver went down my spine as I imagined my sister playing with herself and her new toy. I had already been pressing on the butt plug experimentally, but at her encouragement I wiggled it more purposefully making her gasp softly.

"How did it take me so long to figure out sexy you were?" I asked, not expecting a real answer.

"Because I never had any reason to be sexy for you before," she said. "Now I do. And I think I'm doing a pretty good job."

"God, are you ever."

While I played with Kristin's butt she started grinding slowly on my leg, pressing her body firmly against me. I enjoyed the feeling, but there was an immediate problem to be dealt with if things continued the way they were going.

"You're going to get those panties all wet if you keep that up," I said.

"So? I'm gonna buy them anyway."

"That's fine, but I'm refusing to be seen with you if you set those on the counter soaked and smelling like sex."

Kristin pouted but offered no real objection, particularly when I knelt down in front of her and started pulling the panties down. I brought them up to my face after she stepped out of them and examined the material closely. There was already a small damp spot, but fortunately nothing overly noticeable if you weren't specifically looking for it.

As I set the panties aside I was left kneeling in front of my half-naked sister with her bare pussy directly in front of me. I knew it was an incredibly stupid place for us to risk getting caught, all someone would have to do was open the door and we'd have nowhere to hide. In theory the door was locked, but it wasn't the sort of lock I would trust with anything important. It was just enough to make people feel comfortable trying on other clothes in a somewhat public location. On the other hand Kristin's perfectly smooth pussy was not something I could pass up easily when it was right there in front of me, begging to be touched.

"What you doing down there?" she asked.

"What do you think?"

I ran my fingers up the inside of Kristin's thigh and over her pussy lips, parting them slightly as I did. I gave the length of her slit a preliminary lick and once more experienced the unique taste of my sister, a taste that I absolutely loved.

"You know if I tried stripping you and giving you head in here you'd freak out on me," Kristin said.

"Probably," I agreed. "You want me to stop?"

"Uh uh, no way."

"Good, then stop pointing out my hypocrisy because I'm well aware of it without your help."

I tried to be careful and stay out of trouble, and usually I succeeded pretty well. Kristin was a massive weakness for me in that regard however, I couldn't always do the sensible thing when I was with her. It had always been that way and the effect had only gotten worse recently. Sure I could still maintain control if I really wanted to, but the truth was that more and more I was prepared to choose my sister over any kind of reason or logic.

Kristin's fingers tangled in my hair as I licked repeatedly at her pussy and her legs parted eagerly at my slightest touch. There were other people out there walking by as close as a few feet away, yet I was going to make her cum. It was going to happen. It was one of those rare times when I felt as though I could understand how her world worked, when consequences were meaningless and all that mattered was what I wanted at that moment.

"Mmm, I love it when you get like this," Kristin said. "Everyone thinks you're such a good boy, but you're not really are you?"

I don't know if she expected me to argue the point, but all I did was ignore it. I had more important things to think about, things which would severely undercut any protest I might make to her claim.

Kristin made an appreciative noise as I pushed a finger inside her increasingly wet pussy with a second one close behind. She let me finger-fuck her for a little while before regaining her train of thought.

"What would people think if they knew you as well as I do?" she mused. "Come to think of it, what if they knew just how bad I was too? We'd be grounded forever just for a start."

She giggled at the thought, then abruptly gasped as I used my remaining free hand to find the butt plug she was still wearing. I already knew she liked me playing with it, but I hadn't realized just how much it could affect her when combined with the stimulation on her pussy.

"Oh fuck, keep doing that," she breathed.

Kristin's hips bucked against my face, threatening to dislodge me and making my job significantly more difficult. I knew she was getting close though and I didn't give up, instead responding with more force of my own. I licked furiously at her clit and increased the pace of my fingers inside her until finally she had enough.

With an all to loud cry, at least to my ears, Kristin fell back against the wall as her pussy clamped down firmly on my fingers. I kept licking as she came, only stopping when it was obvious she was through. As I looked up at her in the aftermath her more familiar look of mischief and lust was replaced temporarily by an expression of sated contentment.

The moment was quickly ruined by a knock on the door, making me panic at the returning realization that we weren't exactly somewhere private.

"Are you all right in there?" came a voice from outside.

"Yes thanks," Kristin called back without missing a beat. "Almost done."

"Okay," the voice replied uncertainly.

After a moment I heard footsteps walking away and my shoulders sagged in relief. That was too close.

"Well that was fun," Kristin said.

She bent over and kissed me, evidently not caring that I tasted like her pussy. I felt a twinge of regret as she started getting dressed even though I knew that we had to go or risk attracting more attention. My cock was straining in my pants and part of me wanted nothing more than to pin my sister against the wall and fuck her. Having self-control sucked sometimes.

"Sorry I can't take care of you too," Kristin apologized. "I will later I promise. You more than earned it today."

"Strangely that doesn't really help me right now," I said.

Kristin shrugged and unlocked the door.

"Shall we?" she asked.

She didn't wait for an answer and before I was truly ready I was following her back in to the store proper. My erection had settled down a little, but I was worried it would still be noticeable if someone got too close. Luckily no one was nearby, even the sales girl who almost caught us has been distracted by another customer. At least I assumed it was her, there weren't any other suitable candidates around.

Humming cheerfully to herself Kristin approached the counter to pay for her choice of skirt and panties while I skulked around nearby, trying to watch every direction without making it obvious. By the time we made it out of there I had decided that no one was curious enough about what we'd been up to that they were going make an issue of it and I'd started to relax. Kristin didn't look at all concerned about it the whole time, but I'd come to expect that from her. She had way more practice at acting normal than I did.

"We should do that again some time," Kristin said as we headed back to the car.

"I wouldn't count on it," I said.

"Yeah yeah, you say that now. I could get you back there right now if I really wanted to."

She was probably right, especially given how horny I still was.

"How about we don't test that, okay?" I said.

"Nah, I wouldn't. Maybe if I was bored enough, but I've still got Friday to look forward to."

****

The day couldn't come soon enough for Kristin, as short a time as it was in reality. I watched her get dressed in the morning, as I liked to do when I had the opportunity, and she put far more effort in to her appearance than normal. With her skirt, a white button-up blouse, and her hair pulled back into a ponytail she looked far more like the archetypal schoolgirl than I'd ever seen her before. It only she could have masked the wicked glint in her eyes the image would have been perfect.

I will admit to a certain amount of anticipation throughout the first part of the day. Having sex with Kristin wasn't ever going to be something I would dread, even if the circumstances weren't ideal in my mind. Plus she actually looked really hot in her outfit, in a weird, subtle kind of way since it wasn't overtly any more sexual than something she would wear normally.

By the time fourth period rolled around I was having trouble acting casual and I was sure someone was going to notice that I was behaving oddly. It was a bit of a relief to finally make it to Miss Reid's class with nothing going wrong and finding Kristin there waiting for me. Shannon was with her actually, the two of them deep in conversation until I stepped through the doorway.

"Want to lock that behind you?" Kristin said.

"Sure," I said.

With the door locked we were reasonably secure inside the classroom and either Kristin or Shannon had already pulled the curtains across the windows so no one could see in from outside.

"I told Shannon she could stay if she wanted," Kristin said. "You don't mind too much do you?"

"Uh, no that's fine," I said.

"Good, 'cause we do still owe her for helping us."

The trail of logic on that one was a little difficult to follow. As I recalled Shannon was supposed to be making sure no one interrupted us, which she couldn't very well do if she was watching us. On the other hand having her out in the hallway would hardly be inconspicuous. Fortunately the deal was between my sister and her friend and if it didn't make sense to me it wasn't a big deal.

Kristin stood beside the teacher's desk at the front of the room with her hands clasped behind her back and her chest sticking out slightly. As I approached her Shannon stepped to the side and sat on the edge of one of the tables in the front row.

"See, I knew you didn't really hate this idea," Kristin said as I placed my hands on either side of her waist.

"I rarely hate your ideas, I just don't always think they'll work out," I said. "This time you actually put some thought into it."

I stopped Kristin's response by kissing her, pulling her closer to me as I did. I didn't want to antagonize her enough that she'd up the stakes on me.

My hands slid easily underneath her shirt as we held each other and rested comfortably on the bare skin underneath. I slowly moved one of them upward, over her tummy and not quite reaching her breasts before she pulled away a little.

"Mmm, getting kinda horny now," she said.

"Aren't you always?" I said.

"More than usual then. Here, you can probably feel it."

Kristin took hold of one of my wrists and guided my hand under her skirt to the front of her panties. As I pressed my palm against them I could feel a sizeable damp spot already formed.

"Jesus, we barely even did anything yet," I said.

"I know, but you remember how I said I had another butt plug that's a little bigger? Well I've been wearing it all day and it's been turning me on like crazy. It feels even better than the other one. I think I'm ready for you now, for real this time."

Kristin hips rocked gently on my hand almost of their own volition, and when I pressed back against her pussy she moaned softly. She was definitely worked up already, there was no question about it.

With slightly shaky hands she undid the front of my pants and reached in to grasp my hardening cock. She gave a gentle squeeze and stroked her fingers across the length of my shaft as I continued rubbing her wet panties.

We stood together essentially masturbating each other for a minute or two before Kristin backed away and leaned over the teacher's desk, bracing herself on her arms. She turned her head back to look at me in a silent invitation that I could hardly refuse.

I moved behind her and slid my hands up the outside of her thighs under her skirt, not yet lifting it quite high enough to expose her butt. It was simple enough locating the waistband of her panties by touch and tugging them down to her knees. Kristin let them fall to the floor and carefully stepped out of them, kicking them unceremoniously to the side.

With her panties out of the way I pulled Kristin's skirt up around her waist to reveal her bare ass. I was intimately familiar with this part of her body, except for right in the middle was the black, circular base that was all I could see of the butt plug currently inside her. All I could do for a long moment was stare at the expected yet fascinating sight, too long for my sister.

"Come on, what are you waiting for?" she demanded. "You need instructions back there?"

"Just looking," I said.

To pacify her a little I pressed my fully-hard erection against her pussy, making her sigh happily. She began rocking back and forth in small movements, but didn't have the right positioning to get me inside her without help. Before she got impatient again I guided my cock to her entrance and allowed the motion of her hips to slowly impale her on my shaft. Once I got her started she did all the work and I didn't even have to move.

I heard a sound from behind me and turned my head just enough to glance at Shannon who I had almost forgotten was there. The front of her pants was open and her hand was buried under her panties, masturbating freely at the show Kristin and I were giving her. I'd expected it to happen as she got more comfortable around us, although she was a bit quicker about it than I would have predicted. She met my gaze for a second as I looked at her and seemed embarrassed, but didn't stop what she was doing.

Kristin continued fucking herself on me as I turned back to her, although she faltered slightly as I grabbed the base of her butt plug. As I had done back in the clothing store I merely played with it at first, wiggling it around just to see what kind of reaction I got. My sister had a different idea of what I should be doing.

"You're gonna do it aren't you?" she said. "You're gonna fuck me back there."

"Do you want me to?"

"Duh, that was the whole point of getting ready for you. You think it was just for fun? Actually, it was kind of fun when I think about it."

Kristin giggled softly to herself as I decided on my course of action. It wasn't really a difficult choice when it came down to it, it just seemed like our first time trying anal should have been somewhere a little more traditional. Like in a bedroom.

I pulled on the base of the butt plug, slowly fighting against the resistance Kristin's ass put up trying to keep it in. As the widest part made it past her outer ring she moaned and stiffened up while her pussy squeezed my cock hard. Once I had the butt plug out I waited for Kristin to recover a little before trying anything else. Even after her pussy relaxed it was a couple minutes before she straightened back up to her initial stance.

"You still want me to do it?" I asked softly. "That seemed pretty intense."

"It was kinda," Kristin said. "It's just so... bad having my brother playing with my butt like that. I love it. Although I think if you're not in my pussy at the same time it won't be so much for me and I could handle it better."

"Okay, but you have to tell me if it hurts or if you want me to stop for any reason alright?"

"Promise."

Kristin's asshole was still lubricated from where she'd put in the butt plug and my cock was covered in her pussy juice. I thought about asking if she had more lube with her just in case, but decided to at least try it first and see how easily it went. It wouldn't be completely new to her this time so it might be okay.

I placed the tip of my cock directly against my sister's anal opening and pressed gently, but with gradually increasing force. She pushed back and slowly her asshole reopened for me and let me in. As soon as the head of my cock popped inside her ass she froze and I immediately stopped moving as well. A small growl of pleasure emerged from her and her back arched as far as it would go.

"You still okay?" I asked.

"Fuck yes, this is amazing. Don't you dare stop."

She started rocking back against me, not enough to really drive me any farther into her but just to encourage me to keep going. That was all it took for me since already the feeling of being in my sister's ass was incredible and my instincts were urging me to start fucking her for real.

I couldn't go as fast as I was used to with her pussy of course, her ass was tighter and despite what she was telling me I knew she still wouldn't be ready for anything too rough back there. On the other hand I probably wouldn't last that long even going slowly, not if it continued to feel as awesome as it did.

Alert for any sign of discomfort I thrust my hips forward, pushing a little deeper into Kristin's ass. Unlike regular sex I found my movements had to be more purposeful, more controlled, but it was worth the effort.

"Shit, gonna cum again," Kristin mumbled.

"What already?"

My cock was only a little over halfway inside her, maybe two-thirds at the most, and I hadn't even started to properly fuck her yet. As she had warned me though she was soon cumming again, collapsing forward onto the desk as her orgasm drained the strength from her arms.

You have no idea how good this feels," she said eventually, having recovered enough to speak again.

"I think I'm okay with that," I said.

"You better cum soon or I'm just gonna pass out."

"Alright, but are you-"

"Just fuck me, I'll be fine," she interrupted.

I had my doubts, but I felt the very beginnings of my orgasm approaching and that made it hard to resist too much. Taking a deep breath I thrust into her harder than before, finally burying myself fully in her ass and getting only a soft groan from her. I again had the worry that maybe we hadn't used quite enough lube to allow me to fuck her as easily as I should have been able to, but it was too late to do anything about that.

After a few strokes I found it getting smoother, especially with Kristin helping out a little. She still didn't seem fully over her orgasm and the constant anal stimulation almost certainly had something to do with that, but she still pushed back against me in time with my movements.

It wasn't long before the warning signs became more urgent and it was all I could do to keep up a steady pace until the very end. I didn't even consider pulling out, though I realized after that Kristin wouldn't necessarily want me cumming in her butt. She clearly didn't mind though since she made a noise very similar to a contented purring as the last of my cum shot into her.

I stayed exactly where I was even after I finished, capable of movement but failing to see the appeal. It would have been nice just to fall on a mattress and curl up with Kristin for a while, although that was purely wishful thinking under the circumstances.

It was only when I felt my cock softening that I finally pulled out and stepped back a couple paces, unable to fully look away from my sisters just-fucked ass. A small amount of my cum had trickled out already and I wondered if I should wipe it off. Before I could Kristin stood up and her skirt fell back down to cover her butt.

Once again, since she had been out of my line of sight, Shannon's activities hadn't really been a focus of mine. However her pants were now pulled down around her knees and she had a flushed, satisfied look so I assumed watching Kristin and I had been enough for her to get off at least once. Seeing her state of undress reminded me that I should probably take care of myself and get my cock back in my pants. We all had some straightening up to do and there was no point tempting fate by leaving it until later.

"Could you hand me my panties?" Kristin asked.

They were still on the floor from before, not far from where Kristin was standing but evidently not close enough. I picked them up and handed them over, then watched as she put them on by lifting her legs up rather than bending over any more than necessary.

"Gotta stop all that cum leaking out all over the place," she said.

"Yeah, good thinking," I agreed.

If I hadn't just cum the idea of my cum leaking into her panties all afternoon would have been more than a little interesting. As it was I just filed the thought away for later.

"Well I don't know about you guys, but that may well have been the hottest thing I've ever seen," Shannon said, having pulled her pants back up.

"Mmm, it's definitely a contender for me too," Kristin agreed. "Now I just wish it was the end of the day so we could leave. Don't know how I'm gonna make it through next period, it's gonna seem even more boring after this."

"You always seem to find a way," I said.

"I guess. So hey, me and Shannon were planning on going to her house after school 'cause her parents are gone until tomorrow or something. You want to come too?"

Even in my fully sated state that wasn't a proposition I would pass up right away, not that it was a guarantee anything would happen while we were there. I glanced at Shannon who gave me a small nod and a shrug like it was fine with her but my decision.

"Sure, why not," I said.

"Great." Kristin leaned in to kiss me, then whispered quietly enough Shannon couldn't hear. "I love you, you know."

She turned away and headed out to the hallway with Shannon so quickly I didn't have a chance to respond. I knew Kristin loved me, just as I loved her, but it still made me feel good when she said it.

I sighed to myself as I realized that I too was going to have to sit through the rest of the day, and that there was no way I was going to be able to focus enough to make it worthwhile. Still, there was that evening to look forward to and the possibilities it held. For all I knew having a chance to recover before then might not be such a bad thing.