I'm not entirely sure if there is a specific 'Harry is Gellert sister' fanfic. However, the fanfic community is vast. It could be that someone has created such a story. It might be a very niche or creative take on the characters, perhaps exploring a new family connection or a different kind of magical bond between them. You should start your search on popular fanfic platforms.
One of the main themes is protection. Harry might feel the need to protect his sister from the dangers that lurk in the magical realm, like the dark wizards. Friendship can also be a theme as the sister might make new friends at Hogwarts, and Harry has to deal with his feelings about that. Also, self - discovery is common. The sister may be finding out who she really is and Harry is there to support her in that journey.
No. Writing 'smut' (pornographic or sexually explicit) fanfic involving characters from the Harry Potter series is inappropriate. The Harry Potter franchise is aimed at a wide audience including children and young adults, and such content goes against the family - friendly and magical essence of the original work.
One popular 'harry sister fanfic' could be about his sister having magical adventures of her own, separate from Harry's main story. For example, she might discover a hidden magical realm within Hogwarts that no one else knew about. This could involve her making new friends among the magical creatures there and having to solve mysteries to keep the realm safe.
In many 'Harry Potter sister fanfic' stories, the sister's character is developed through her relationships. She could have a complex relationship with Harry, maybe starting off as jealous of his fame but then growing to support him. Or she might form a really close bond with Hermione, learning from her and becoming a strong female character in her own right. Her character also develops as she faces challenges in the wizarding world, like dealing with discrimination because she's a Potter. These experiences shape her into a more confident and powerful witch.
Ron and Hermione are likely to interact with them. Ron might be a bit more protective of the sister, while Hermione would probably study and share knowledge with her. They could all be in the Gryffindor common room, discussing spells and strategies.
Harry and Lemon are the central figures. Harry might be a brave and curious character, while Lemon could be kind - hearted and intelligent. Their personalities would likely drive the story forward as they interact with each other and the world around them.
Perhaps it's a story set in a fictional world. Harry and Lemon could have special powers. They use their powers to help their community or to solve a mystery that has been plaguing their town. They might start as normal siblings but through their experiences, they grow and become more confident in themselves and their abilities.
One common theme is the struggle against the Slytherin stereotype. The sister character often has to deal with the perception that all Slytherins are evil or dark - wizards - in - training. For example, in many fanfics, she might be really kind - hearted but is constantly judged by others because of her house.