Sibling Love (Fate/stay night)

A/N: Sibling Love was a commissioned one shot originally written back in December of 2020! Posting it up here and now for people to enjoy.

Themes: Loving Sex, Threesome, Bathtub Sex

-x-X-x-

He's just making dinner for one, albeit with plenty of leftovers for the ravenous women in his life, when the front door suddenly swings open with a bang. Blinking, Shirou looks back over his shoulder just in time to catch the tail end of his adoptive big sister, Taiga Fujimura, disappearing through another doorway.

"Taiga? You're back early! How was the date?"

His hesitant callout is met with silence at first, and Shirou frowns, torn between keeping his attention focused on the meal in front of him, and checking on Taiga. On the one hand, he loves her as if they were really siblings, feeling no end of affection for the older woman. On the other hand, his love for cooking was very nearly as great, and there were a couple of people, Taiga included, who would be quite agitated if they didn't have access to leftovers of one of his dishes on the morrow.

But no, in the end Taiga is more important than any meal, at least to him. Turning off the burner he was using and setting down his spatula, Shirou goes to take off his apron as he turns around, aiming to track down his Big Sister… only to run smack dab into her instead, the older woman having snuck up on him while he was distracted with his inner thoughts.

Cradling a bottle of sake in her arms and staring at him with wide, watery eyes and a wobbling lip, the so-called Tiger of Fuyuki bursts into tears, balling.

"He stood me up, Shirooooou! Waaaaah!"

Shoulders slumping, Shirou can only hug his big sis close with a sigh.

"Fuji-nee…"

It was an unfortunate fact of life that sometimes, love was hard to find. Sometimes, it feels completely nonexistent. As Shirou does his level best to be there for Taiga, the meal he was preparing goes unfinished. This isn't a normal rejection, it would seem. It's the straw that broke the camel's back, and even all of Shirou's attention and comforting doesn't stop Taiga from getting drunker and drunker as she vents at him.

Only once does Shirou try to suggest that he could make her something to eat to feel better, hoping to get back to the kitchen before his more precious ingredients go bad. But in a manner that tells Shirou just how bad this is, just what sort of dire straits his adoptive sister is in, Taiga rejects the offer, saying she has no appetite, begging him to stay by her side.

It's a testament to just how broken she is by this most recent rejection. Though, as Taiga explains to Shirou through her venting, it's not just about this particular rejection. Being stood up by this random guy, his big sister says, isn't in and of itself that bad. But it represents a larger pattern of rejection, of failure to find love. It really is the last straw for Taiga, who in between ranting and snarling like the Tiger she's often compared to, constantly bemoans her nonexistent love life and wonders if she'll ever be able to find true companionship.

By the time she finishes off the bottle of sake, Taiga is well and truly sloshed, and even she knows it, looking down at herself in a drunken stupor, blinking blearily.

"Yeesh… I need a cold shower or something… hic!"

Shirou nods sympathetically, his eyes trailing over to the kitchen for the briefest of moments before giving it up as a lost cause. It's been too long by this point, even if he could get back in there, he'll have to scrap what he'd already made and start over anyways. With that knowledge, his shoulders slump a little and he offers Taiga a soft smile.

"I could go draw you a bath, if you like Fuji-nee."

Taiga's eyes light up at that, and she's suddenly sitting a bit straighter as she looks at him with a grin.

"Yes! Yes, do that! It'll be just like old times!"

As Shirou stands up and begins moving to the bathroom to do exactly that, he smiles fondly. It wouldn't be quite like old times. He remembered, when he was young, bathing together with Taiga. As his adoptive big sister, she was always there to help him wash up. It was just something that siblings did. But of course, after Shirou hit puberty and started to grow up, their communal bath time quickly grinded to a halt.

Shirou couldn't remember why, exactly. Maybe Taiga realized she was having an effect on his young body before even he did, or maybe she just decided it had grown inappropriate, after a certain point. Either way, it'd been years since they'd done anything like that, years since Taiga and he had taken a back together.

It had not been quite so long since Shirou had drawn a bath for his drunken big sister, however, and that's what he does now, filling the tub with water and even some bubbly soap for good measure. Once he's got it to a nice, warm temperature, Shirou leans back and straightens up, preparing to turn and get Taiga in here before he leaves. Maybe he can scrap his current kitchen project and even make something small while Taiga is bathing or-

Shirou never gets a chance to finish that thought, because as he turns around to go and get Taiga, it's to find she's once again snuck up on him while he was deep in thought. This time however, the drunken Tiger of Fuyuki is completely naked, her face and chest flushed with the sake she'd downed as she blinks at him just as owlishly as he blinks at her.

"Huh? Why're you shtill dressed, Shirou? Shtrip down and get in the tub little bro… hic!"

Advanced upon by a stubborn, drunken Taiga, Shirou has no choice. He very studiously keeps his eyes up above her neckline as he strips down himself and climbs into the tub. It's awkward and tense for a moment while Taiga does the same, sliding into position behind him… and then her hands come down on her back and Shirou finds himself drawn back to those days long past.

Bath time with his big sister was actually always something of a comforting time for Shirou. As embarrassing as society dictates it should be now at their ages, it still feels rather nice.

"I'll wash your back, Shirou, just like old times~"

"Okay, Fuji-nee…"

Resting there in the tub, feeling Taiga running her hands up and down his back… it feels good. It really does. Letting his eyes drift shut, Shirou is able to really forget about the mess in the kitchen for the first time since Taiga came home, just living in the moment and enjoying her hands rubbing across her back.

Taiga, meanwhile, drunk as a skunk, finds herself transfixed by how broad her little bro's shoulders have gotten, and how strong and powerful his back muscles feel, rippling as they are beneath her questing hands. Licking her lips, the school teacher forgets just who she's dealing with for a second and finds herself getting aroused by just how… masculine the man sat in front of her truly is.

Of course, a moment later she remembers that the 'man' in question is Shirou, even in her drunken state. But… so what? Shirou has really grown into a fine young man. That's not a bad thing. In fact, it's quite good. Really good…

"Shirou…"

"Yes, Fuji-nee?"

"T-Turn around. I need to wash your front now too…"

Shirou's back goes ramrod straight at that, his muscles rippling even more before her eyes as he looks back over his shoulder at her, visibly concerned.

"I can handle my front just fine, Fuji-nee…"

Shaking her head back and forth adamantly, Taiga gives Shirou her best stern older sibling glare.

"No. I'm gonna do it. Sho, turn around!...hic!"

Reluctantly, Shirou does as he's told, proving he's still the good little brother that she knows and loves. Except… there's really nothing little about her brother anymore. He's practically her BIG brother now in all the ways that matter, Taiga can't help thinking with a drunken giggle as she looks down at him. There, beneath the water, past the remaining few bubbles, she sees his cock.

It's fully erect right now, an example of why Taiga felt compelled to stop these communal baths back in the day in the first place. All those years ago, she'd been afraid of going too far, of giving Shirou a sister-complex or something, of ruining him and ruining his ability to form good, meaningful relationships with other girls.

But fuck that. Fuck society's definitions of romance and love. Fuck their social taboos. Taiga's hands start on Shirou's chest, and she greatly enjoys the feels of his pecs, of his muscles beneath her questing fingers. But her eyes fixate on his big manly cock and stay there the entire time, until eventually her hands catch up to it. She finds herself grabbing it and squeezing it and groping it as Shirou lets out a wanton groan in response.

Not even looking, Taiga reaches back and pulls out the stopper in the tub, draining most of the water so that his cock is finally breaking the surface. As soon as it's no longer underwater, Taiga dives down and gives it a kiss, just a peck on the head, leaving Shirou to grunt as he jolts in place.

"Fuji-nee, we really shouldn't be doing this…"

And yet, Shirou doesn't resist much harder than that. Not only can he not bring himself to raise a hand against his big sister… this is a recreation of his very first sexual fantasy. Taiga hadn't ruined Shirou like she feared, but he had developed something of a minor sister-complex before those baths stopped all those years ago.

Now, with Taiga reawakening it within him, Shirou was having to fight against his own desires, leaving him with precious little will left to fight off Taiga too. Looking up at him from where she's leaning forward in the tub, the Tiger of Fuyuki just grins a drunken grin.

"Don't worry, Shirou. I'm just… just dishcharging my duties as your big sister~ Just let Fuji-nee take care of you~"

And then she leans forward again and is right back to putting her mouth on his cock, licking and lapping at it now, slurping and slobbering all over his dick. Another loud groan emits from Shirou's mouth and he tilts his head back, unable to fight it any longer. It didn't take much to be fair, but Taiga has ultimately convinced him to go along with it.

Finally, she takes his cock into her mouth directly and begins to suck on it, lewdly and wantonly. In her drunken state, all Taiga knows is one thing… Shirou is the only man who's ever been there for her. He's always available, the only masculine presence in her life who hasn't abandoned her or left her or rejected her. For that, he deserves a reward. He deserves ALL of her effort.

Needless to say, the ensuing blowjob is undeniably… messy. How can it not be, when Taiga is drunk as she is? With a whole bottle of sake flowing through her, the school teacher is nevertheless very enthusiastic about sucking Shirou's cock, bobbing up and down his length as she plants her hands palm down there in the tub, her eyes gazing up at his face.

Watching him groan and grunt in pleasure at the attention she's showing him only spurs the drunken woman on even further, her tongue swirling all over his cockhead and her lips suctioning down on his shaft as she sucks and sucks. Even her cheeks suck in as she does her level best to use her mouth like a vacuum around his member. Finally, all of her hard work seems to pay off.

"Fuji-nee… I'm getting close!"

Hearing that, Taiga can do nothing else but redouble her efforts. Swallowing his dick to the hilt more than once in her drunken state, she's still caught a little by surprise when he suddenly begins to cum, his seed causing her cheeks to puff out like a chipmunk for a moment before she catches herself and begins to swallow. Drinking down his entire load without missing a drop just makes sense to Taiga's inebriated mind, so that's what she does. His cum is salty, but nevertheless doesn't taste all that bad.

As she's swallowing Shirou's cum, Taiga comes to a realization of sorts. She loves him. She loves Shirou Emiya, not just as a brother, but also as a man. Her perfect man, her perfect romance partner has been there all along… in the form of her little brother, who's not so little anymore.

Pulling back off of his cock with a pop once he's done cumming, Taiga looks down and is so very pleased to see that Shirou is still rock hard, his youth allowing for a very short refractory period. With him ready to go again and still dazed from his last release, Taiga straightens up and shows off her body, running her hands up and down along her front as she beams at him drunkenly.

"Do you like what you see, Shirou? Do you want some more of me?"

Still a little out of it, Shirou just nods, sort of drunk in his own fashion, drunk on his big sister and her gorgeous body. Grinning widely, Taiga climbs aboard Shirou's cock, moving into his lap as his powerful, manly hands go to her body, holding her tight. In turn, the drunken older woman wraps her arms around her not-so-little bro's neck and leans in to give him a kiss.

As she locks lips with him, she also lowers herself onto his dick, groaning into Shirou's mouth as he does the same from the pleasure of them finally being joined. His cock fits so snugly inside of her that Taiga inwardly beats herself up for not doing this before now, for not realizing that the perfect man was stood in front of her all along.

As brother and sister become one, Taiga pulls her lips back from Shirou's, tossing her head back as his mouth instead falls to her breasts. As she cries out in ecstasy, she also calls out to him, confessing to the one man who will always be there for her.

"I-I love you, Shirou! Please, please fuck me! Make me your woman! Turn your big sis into your wanton slut!"

"Fuji-nee…"

There's a note of consternation in Shirou's voice as he thrusts up into her from below. They both know Taiga's drunkenness is just getting the better of her, and that he would never ever be able to think of her as a slut. Giggling in a silly fashion, Taiga just buries her face in Shirou's neck to try and contain her moans, holding onto him for dear life as she bounces up and down on his cock, riding him towards kingdom cum.

It feels so good to finally give in to their feelings for both adoptive siblings. For Shirou, this is the fulfillment of a fantasy he's had since he was but a boy just experiencing the opening effects of puberty. For Taiga, it's finally finding love from a man she knows won't leave her, won't reject her. Shirou loves her unconditionally, taking care of her at her lowest points and celebrating her at her highest.

He already knows the worst about her, already knows what she brings to the table, and he accepts her with all his heart anyways. What's not to love about that? What's not to desire? Mewling pitifully, Taiga cums hard around Shirou's cock, a combination of his thick fat prick digging deep into her gushing wet cunt as well as her own thoughts, driving her right over the edge.

She moans and shakes and trembles atop him, while Shirou just holds her close, luxuriating in the feel of her soft body pressed against his hard one, his tongue continuing to trace over her tits, which are still in his face, right below his chin. Slobbering and sucking at her breasts with somewhat reckless abandon, Shirou doesn't let up for even a moment.

Finally though, his next release begins approaching. Taiga has cum a second time by now, and her clenching pussy walls are what are rapidly drawing him closer and closer to the edge. As he groans loudly, not sure how much longer he can hold on, Shirou does his best to warn his big sister about what's coming.

"Fuji-nee, I'm getting close again!"

Taiga doesn't even hesitate.

"I-Inside! I want it inside… hic!"

Whether out of sheer love or sheer drunkenness, Taiga makes the call and Shirou doesn't have it in him to protest her decision or try to countermand it. Buried deep, DEEP inside of his big sister's hungering cunt, the young man lets out a wanton groan and proceeds to spill his seed in her depths, filling her with a nice, thick, creamy load of white spunk.

They both lock up for a moment at that, freezing in place briefly before slumping against one another. Just as Taiga is slumping into Shirou and he's tightening his grip on her in order to support her and prop her up… the door to the bathroom opens.

Illyasviel von Einzbern, Shirou's OTHER adoptive sister, stands frozen in the entryway clad only in a towel, her red eyes wide on what she's just walked in on.

Taiga and Shirou both look to her, eyes equally wide, caught in the act of forbidden love, a love that technically wasn't truly taboo since they weren't blood related, but would no less be frowned upon by all of society if proof of their indiscretion were ever discovered.

-x-X-x-

Luckily for all of them, Taiga is still quite drunk. And Drunken Taiga is prone to having epiphanies and revelations at the drop of a hat. The way she sees is, Shirou and Illya are ALSO brother and sister after a fashion, right? And if Taiga and Shirou could share such unadulterated love with one another, then why shouldn't Shirou and Illya share their sibling love as well?

"Illya! You're just in time! Shirou and I were just consummating our relationship, not just as brother and sister, but as lovers! And you should too!"

Shirou stiffens beneath her in more ways than one, his cock leaping within her clenching cunt and his body going ramrod straight in Taiga's arms. But Illya, who has been staring at the scene in abject shock, processes Taiga's words and begins to develop a tentative, painfully shy smile, that slowly transforms into a wide, wicked grin of delight as Taiga gives her an encouraging smirk in return.

"Okay!"

Without hesitation, the small white-haired woman sheds her towel and all but skips over. Despite having a fairly petite body, Illya was actually technically older than Shirou. By all accounts, she was his 'big sister' as well. Except, normally their dynamic was more big brother, little sister. Well, in this moment, as Taiga leans back out of the way, Illya certainly takes charge as the big sister just a tiny bit, positively glomping Shirou and grabbing him.

Wrapping her small arms around his neck, the petite homunculus kisses her adoptive brother right on the lips, clearly hungry for it, possibly even hungry for Shirou in particular. Because really, Taiga had never gotten the impression that Illya was this sort of randy before… which meant it HAD to be Shirou who was causing the change in her!

Smiling drunkenly and nodding to herself in approval at having figured it out, Taiga pulls herself completely off of Shirou's cock. As Illya leans over the edge of the tub to kiss the ever living daylights out of Shirou, Taiga reaches out and wraps her hands around the white-haired girl's pale body and lifts, pulling her up and over the bathtub rim and into the tub itself.

"Let me show you how it's done, Illya~"

As the two sibling lovebirds are swapping spit, Illya doing her level best to shove her tongue down Shirou's throat, Taiga gets into position behind the younger woman, pressing her larger breasts against Illya's back as her hands snake around to Illya's front. One hand grabs hold of one of her much smaller tits, while the other slips between Illya's legs. As Taiga begins to slide her fingers along the younger woman's pussy lips, she whispers into Illya's ear, though more than loud enough for Shirou to hear as well.

"Don't worry about me, Illya. I'm just getting your tight, virgin sister-pussy ready for Shirou's big fat cock. You've gotta be nice and wet down there or else he'll hurt you, and we don't want that!"

Illya whimpers into Shirou's lips and shakes her head in agreement with Taiga's words. Shirou, meanwhile, groans into his 'little sister's' mouth, his cock throbbing and pulsating, banging up against Taiga's fingertips and Illya's undercarriage as he gets hopelessly turned on by Taiga's terminology. Grinning wickedly, Taiga doesn't let up, not even for a moment. She gives Illya the preparation she wished she'd had on HER first time. The dude Taiga had lost her virginity with had been a completely forgettable lout.

He'd been rather small dicked, but big enough to tear her hymen and leave her with a painful and sore experience after cumming in maybe two pumps. That first time had been the beginning of a long string of disappointments, when she wasn't getting stood up to begin with, or rejected out of hand by guys who couldn't handle her aggressive personality.

But not only could Shirou handle her, his cock was nice and thick and fat, and it'd felt amazing in her own tight wet cunny. In comparison, Illya was so much smaller than her, and Taiga would be damned if she let the brother and sister duo have a bad first time together.

Eventually, she decides that Illya is good and ready, the petite young woman's pussy lips practically drooling all over Taiga's fingers by the time she's satisfied. Removing the hand from Illya's breast, Taiga instead slides it down to Shirou's cock, using her fingers to splay Illya's pussy lips open as she guides Shirou's dick up into it.

As Illya sinks down her 'big brother's' length, she tosses her head back just like Taiga did and moans wantonly. With her head laid out on Taiga's breasts, the drunken older girl gets to watch from above (and upside down) and Illya's face contorts in pleasure, her red eyes becoming hooded, her mouth forming into an o of ecstasy.

Shirou, meanwhile, groans as well, and proceeds to bury his face in Illya's tits. They're much smaller than Taiga's, but if that bother's him, the red headed young man doesn't say it, his mouth and tongue working over her pale breasts. This continues for a time, until eventually they're kissing again.

Taiga, meanwhile, remains right where she is, and once Shirou is no longer paying special attention to Illya's bust, she pulls her hands up to those cute little titties and plays with and fondles them from behind the other girl to her heart's content. All the while, she gives her sisterly encouragement, her drunken perverseness infecting the both of them… not that they needed much encouragement to really give in to their true, deep desire for one another.

"That's it. Ride him, Illya. Ride his cock, show him that you belong to him. Let your big brother make you his woman, just like he did me!"

"Y-Yes! Onii-chan, d-don't stop!"

"I won't, Illya! I promise!"

Shirou's heartfelt words are followed by him holding her by the hips and slamming up into her gushing cunt again and again. The pale, white-haired young woman cums over and over around his cock, seeming to have a hair trigger in comparison to Taiga's own body. Her squeals and shrieks of ecstasy fill the bathroom, and luckily there's no one else in the household for them to alert, because Illya might just wake up the whole neighborhood with her orgasmic cries.

The way her smaller body tenses and flexes in Taiga's grasp as she bounces up and down on Shirou's cock, riding him to kingdom cum, is honestly a bit awe-inspiring. Taiga can only imagine how tight she is around Shirou, how her pussy is gripping down so very hard on his prick. Shirou himself is definitely groaning up a storm, his eyes drifted shut and his mouth open in a constant moan.

The older of the three can only watch in awe and amazement as Illya seems to explosively orgasm every few seconds, until finally, Shirou simply can't hold back anymore.

With Illya in his lap and Taiga pinning her to him, there's nowhere Shirou can go. In the end, he can't do much beyond let out a strangled groan as he cums inside of Illya as well, seeding her just like he seeded Taiga. Not that the other girl minds one bit, a wanton moan leaving her own lips as she enjoys getting creampied by her big bro.

As they all luxuriate in the bathtub together, Taiga eventually reaches back and puts the stopper back in, before refilling the bath. What follows is an actual communal bath between the three of them, far more in line with the old days where she had bathed Shirou when he was much younger. Except, this time there's undeniably a charged sexual undercurrent to the whole thing, with playful and affectionate touches between lovers happening here and there as they all cleaned one another off.

This, in turn, led to them all ending up in Shirou's room, in his bed afterwards for even more sibling loving. Still quite drunk, Taiga is all for it and eagerly encourages it, from Illya sucking at her tits and eating her out while Shirou fucked her, to Taiga doing the same for the smaller woman while in turn getting fucked herself.

All in all, it's a glorious night for all three of them, and they end up falling asleep only after they've all fully exhausted themselves, drained from the night's activities. In a bit of a cuddle pile, they fall into a complete and restful slumber.

-x-X-x-

Waking up the next morning isn't quite a pleasurable experience for Taiga. She definitely has the headache of a hangover, and she knows instantly that she drank too much last night. However, she doesn't remember what happened AFTER drinking a whole bottle of sake… until she finally opens her eyes fully and looks about herself.

The sleeping forms of Shirou and Illya curled up against her bring back to mind all the memories of the night before, and the big sister and school teacher is entirely mortified at what she's done. Not only had she forced her little brother to take a bath with her, but she'd also then proceeded to suck and fuck him against his objections.

And THEN, as if that wasn't all bad enough, she'd gone on to encourage him to fuck HIS little sister too! Taiga knew that Illya was technically older than Shirou, but it didn't matter! Their dynamic was such that Shirou might feel terrible about what he'd done when he woke up, and it would be all Taiga's fault!

Just as she's wallowing in her self-doubt and self-recrimination, something alerts the still sleeping forms of her bedmates. Clutching at her all the tighter, both Shirou and Illya mutter under their breaths one after the other.

"Fuji-nee…"

"Onee-chan…"

Looking at them both, biting her lower lip… Taiga hugs them closer to her naked bosom and lets out a low, shuddering sigh. This… this was fine. She wasn't going to second guess herself any further. She wasn't going to let society tell her what was right and wrong, and if Shirou beat himself up over it, she'd tell him the same.

They're both her little brother and sister. Maybe before now Illya and she were only connected through Shirou, but this went beyond that. Taiga was officially adopting the younger woman as her little sister, which meant they were her younger siblings! So it was fine if she loved them both, both as siblings… and as lovers.

It was all perfectly fine.

-x-X-x-

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