The older sister could also have a different relationship with Snape. She might see through his tough exterior and befriend him early on. This could change the dynamic between Harry and Snape as well. Since she's older, she could have more influence in the wizarding world and use it to Harry's advantage.
Well, perhaps the older sister could be a powerful witch. She might protect Harry from the Dursleys' mistreatment in a more forceful way than in the original story. For example, she could use magic to make sure Harry has enough food and a proper room.
In the fanfiction, the older sister could be a member of a secret organization against Voldemort. She could train Harry in magic that Dumbledore doesn't teach him at Hogwarts. Maybe she has some ancient spells that she passes on to Harry, which gives him an edge in the fight against the Dark Lord.
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.
The plot might revolve around the older sister being a part of a prophecy too. Just like Harry, she has a special destiny. But instead of being in the spotlight like Harry, she works behind the scenes to ensure Harry's success in the fight against Voldemort. She could be the one who finds out about Horcruxes first and guides Harry in that direction, all while dealing with her own challenges at Hogwarts, like trying to keep her identity as Harry's sister a secret from the wrong people.
In the fanfiction, Hermione and her older sister could have a complex relationship. Maybe the sister is overprotective, constantly looking out for Hermione at Hogwarts, especially with all the dangerous situations she gets into. The sister could also be a bit jealous of Hermione's academic success and magical abilities, which could lead to some sibling rivalry.
Maybe the older brother could be a protector. He might use his magical skills to keep Harry out of trouble at Hogwarts. For example, when Harry faces difficulties with Snape, the older brother could step in and smooth things over.
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.
In a fanfiction, Harry Potter's older sister could have been a key figure in the fight against Voldemort all along. She could have been secretly trained by some powerful wizards in hiding. For example, she might have been mentored by Dumbledore in a very covert way, learning advanced magic that even Harry didn't know about at first. This would give her a unique role in the overall battle against the Dark Lord.
She could be involved in the Akatsuki plot in some way. In the fanfiction, she might have been sent on a mission that led her to cross paths with the Akatsuki members. This could create a whole new storyline where Naruto has to rescue his sister from the clutches of the Akatsuki, adding more depth to the Naruto universe.
The older sister could act as a mentor. Since she's older, she might have more experience at Hogwarts and could guide Hermione through the social and academic aspects. For example, she could tell Hermione which professors are more lenient and which ones to be extra careful with.
She could play a protective role towards Harry. Since she's older, she might feel a strong sense of responsibility for her younger brother. For instance, she could be the one who first discovers some of the secrets about Voldemort's return and warns Harry.
The sister might have her own group of friends in the wizarding world. When she takes in Harry, she brings him into this circle. They all contribute to his upbringing. For example, one friend could be a potions expert and teach Harry about potions in a fun way. Another could be a great dueler and train Harry in combat magic. His sister coordinates all this, ensuring Harry gets a well - rounded magical education and a loving family environment.