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.
There are likely fanfics that focus on the relationship between Issei and his sister in a more family - friendly and positive way. You can start by looking on popular fanfic websites like Archive of Our Own (AO3). Search for tags related to Highschool DxD and Issei's sister. Some might explore her character development, how she copes with Issei's new life as a devil, or their past memories together. These types of fanfics can offer interesting insights into the characters without any inappropriate content.
One idea for a fanfiction about Issei's birthday could be that the Occult Research Club throws him a surprise party. They decorate the clubroom with all his favorite things, like pictures of his favorite heroes and a big banner that says 'Happy Birthday Issei'. Rias could present him with a special gift, perhaps a new piece of sacred gear that has been customized just for him.
In a fanfiction, it could be that Koneko and Issei are chosen for a special mission by the Gremory group. They are initially hesitant to work together as they have different approaches. However, as the mission progresses, they start to complement each other's styles. Koneko's stealth and Issei's more upfront fighting style work in harmony to defeat the enemy and complete the mission, and in the process, their relationship grows.
One great fanfiction idea could be that Issei discovers a new power within himself that is directly linked to Rias's heritage. Maybe he can tap into an ancient magic that only works in harmony with Rias's own powers. As they train together to master this new ability, their relationship deepens beyond just the typical master - servant bond in the Highschool DxD world.
One great fanfiction idea could be Issei and Irina going on a special adventure together in the world of Highschool DxD. Maybe they discover a hidden power or a new realm. They could face challenges that test their relationship and grow stronger as a result.
In a fanfiction, Issei and Koneko could be paired for a school project. At first, they struggle to work together because of their different personalities. Issei is more outgoing and impulsive, while Koneko is quiet and reserved. But as they dig deeper into the project, they start to understand each other better and develop feelings for one another.
A fanfiction idea could be that Issei and Xenovia get transported to a different dimension. In this new world, they are separated from their friends and have to find a way back. Issei tries to keep Xenovia's spirits up as they encounter various strange creatures. They discover new powers within themselves during their journey. Xenovia might also learn more about Issei's pervy side and they have some comical moments while still trying to figure out how to return home.
The fanfic might also explore how Issei's powers are affected by the transformation. In the original Highschool DxD, Issei has his own set of abilities as a male. But as a girl, he could either have new powers related to femininity in that universe or his existing powers could be modified. This could lead to new battles and challenges where he has to learn to use his powers in a different way.
There's a fanfic called 'Rias and Issei: A New Adventure'. In this one, they go on a journey together that tests their relationship. The author does a wonderful job of portraying their feelings for each other and the challenges they face as a couple in the world of Highschool DxD.
I really like the fanfic 'Koneko's Heart for Issei'. In this one, it delves deep into Koneko's emotions towards Issei. It starts with her being a bit reserved but as the story progresses, her true feelings start to surface. Another interesting fanfic is 'Issei and Koneko: United'. This fanfic shows how they work together to face challenges in the Highschool DxD world, and their relationship blossoms as they rely on each other.