One interesting story could be about how Harry's sister discovers she has magical powers in a very unique way, different from Harry. Maybe she finds out she can communicate with magical creatures that most wizards can't. This would add a new layer to the wizarding world in the fanfic.
In some fanfics, the sister could be a key to unlocking an ancient magic that has been forgotten. Her presence in Hogwarts might stir up old secrets and prophecies. The story could revolve around how she and Harry work together to figure out this new magic and use it to defeat the dark forces.
It changes the family dynamic. Instead of being an only child, Harry now has a sister, which means there's more family support or potential family drama. For instance, the sister could be a target of Voldemort too, adding another layer of danger.
One possible plot could be that Draco's older sister, let's call her Astoria for example, starts to develop feelings for Harry while trying to bridge the gap between the Slytherin and Gryffindor houses. She might see the good in Harry that Draco refuses to acknowledge at first. This could lead to a series of events where she has to choose between her family's traditional views and her new - found respect and love for Harry.
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.
Their relationship might start because of a shared interest in a particular subject, like Defense Against the Dark Arts. Maybe they both have unique insights on a certain spell or dark creature, and that sparks a conversation which leads to them getting to know each other better.
Maybe the twin sister is sorted into a different house than Harry. This would cause a bit of a rift at first as they have to adjust to different house cultures. But in the end, they learn to combine the strengths of their respective houses to overcome a great threat to Hogwarts.
She could be a half - blood like Harry. Maybe her mother was a muggle - born witch and her father was a pure - blood wizard. This mixed heritage could give her a unique perspective on the wizarding world's issues regarding blood purity.
Often, she starts as a naive and somewhat scared character, growing up in a world full of danger. But as she faces challenges, she becomes more brave. For example, she might initially be afraid to use her magic in public for fear of being caught by Voldemort's followers, but later she uses it boldly to save others.