Finding a fanfic specifically about Issei's sister in Highschool DxD with a lemon theme is not only difficult because of the potential adult nature but also because it may violate the integrity of the original story. Highschool DxD already has a complex and interesting plot with many characters. Fanfics usually expand on the existing relationships and storylines in a more positive and appropriate way. So, while there may be some creative interpretations of Issei's family relationships, a 'lemon' - themed one is likely to be rare and not in good taste.
I'm not sure if there is a specific fanfic exactly like that. Fanfics can vary widely, and while there might be some that involve Issei's sister in the Highschool DxD universe, a 'lemon' themed one might be harder to find or might not be in line with the general community guidelines. It's important to note that 'lemon' usually implies adult - themed content which is often restricted in many fanfic platforms.
Chapter 515, "Dismantling A Structure", is the clearest early answer because it turns Damien's situation into a concrete plot event. Early on, Not the kind of quiet that came from peace, but the kind that.... By the end, For a long moment, everything felt… normal.. Inside the broader faction conflict storyline, the chapter closes with And the purge was about to begin., which gives the current arc a stronger narrative position and pushes the power growth onward.
Chapter 122, "Finally Started to Withdraw【Three in One】", is the clearest early answer because it turns Xia Hong's situation into a concrete plot event. The chapter turns that into something concrete by moving from The broadsword can be used, but against a swarm of rodents, its... to The lead two rodents were nailed by the Iron Arrows, the poison sacs....
Chapter 121, "Ready for Everything, Time to Take Action [Three in One]", is the clearest early answer because it turns Xia Hong's situation into a concrete plot event. A useful checkpoint is the way the scene starts with After Xia Hong finished speaking, he gestured for Xia Chuan to rest... and follows through with Even Xia Chuan and Luo Yuan, the six in the Earth-digging Realm, had....
Chapter 157, "The Disappearance of the Cold Beast, Breakthrough Tide", is the clearest early answer because Yuan Cheng finally turns buildup into a real advancement. Early on, The Great Xia Station, second floor of the wooden house.. By the end, Nine days ago, when Hong Mu's team cut down a tree, it even alerted....
Chapter 123, "Arrow Bamboo Forest, Rat King, New Camp", is the clearest early answer because the buildup finally cashes out in a real clash. The chapter gives readers a real answer by opening on Facing such an overwhelming victory, Xia Hong couldn't help but feel... and ending on Not to mention, it shot out directly from underground!. In story terms, the chapter closes with At this moment, Xia Chuan had placed the unconscious Luo Yuan on the ground.
Chapter 481, "Didn't Protect Your Hand", is the clearest early answer because it turns Damien's situation into a concrete plot event. Early on, Seliah, drenched in sweat but alive, called out as she speared.... By the end, Always happy when demons show up to be eaten." The soldiers struck.... Inside the broader battle storyline, the chapter closes with A few soldiers snorted. Others openly laughed. Several nobles turned pale. A handful stepped back, suddenly nervous to even be near Damien.
Chapter 46, "After modification: Chapter 46 Wrong, there's an issue [Please follow!]", is the clearest early answer because it turns Xia Ding's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Hearing Shi Qing's polite request to leave. and follows through with Connecting these clues, Xia Hong's face turned drastically as he....
Chapter 19, "The Battle of the Mane Wolves", is the clearest early answer because the buildup finally cashes out in a real clash. The chapter turns that into something concrete by moving from As the sound of beast hooves arose, Xia Hong and the others hurriedly... to Green blood oozed from its abdomen, and the Snow Mane steadied.... What makes it stick is that it turns Snow Mane's growth into a concrete upgrade step and sets up the next payoff in the power growth storyline linked to Frost Wolf.
Chapter 472, "Galandra Is Yours", is the clearest early answer because it turns Damien's situation into a concrete plot event. It opens with Damien had grown used to eyes following him wherever he went, but... and closes on What," he said flatly, "have you brought into my office?" Haldric.... The key payoff is that the chapter closes with They believed he was a beast tamer rather than a summoner and he would have them hold on to that belief until whenever.
Chapter 20, "Got It", is the clearest early answer because it turns Snow Mane's situation into a concrete plot event. Early on, Having seen the Snow Mane feign death once before, Xia Hong naturally.... By the end, That torch could heal the Snow Mane and even make it stronger.. Inside the broader battle storyline, the chapter closes with "Got it!, which gives the main cast a stronger narrative position and pushes the power growth onward.