Voldemort's remaining followers could be antagonists. They might target Harry's little sister thinking they can use her to get to Harry. They could try to kidnap her or use dark magic on her.
In a 'Harry Potter and His Little Sister' fanfiction, the sister could be a new student at Hogwarts when Harry is in his later years. She struggles to fit in because of being Harry Potter's sister. Harry has to help her deal with the pressure, make friends, and also face the threat of a new dark wizard who has a vendetta against the Potter family.
One common theme is family. Since it's about Harry having a little sister, it often explores the family dynamic. How they support each other, protect one another, and sometimes even have sibling rivalries. Another theme is identity. The little sister has to figure out who she is in the shadow of her famous brother.
Well, in fanfiction about Harry Potter having a little goblin friend, it could be really interesting. Maybe the goblin friend is an outcast in the goblin society and Harry befriends him. They might go on adventures together in the Forbidden Forest, looking for rare magical plants. The goblin could have special knowledge about the forest that Harry doesn't. And they could also form an alliance against the Death Eaters or other threats in the wizarding world. It would add a whole new dimension to Harry's experiences in Hogwarts and beyond.
One possible reason could be jealousy. Maybe the sister is getting more attention from their parents or at Hogwarts. For example, if she is a star student and Harry feels overshadowed.
Obviously, Harry Potter himself is a central character. His sister would be the new addition. Hermione and Ron are likely to be there as well. They are Harry's best friends in the original series and would probably play important roles in the fanfiction too. Dumbledore might also appear, as he is a very important figure in the wizarding world and could guide the sister in her magical journey.
If such fanfiction exists, it may lead to a lot of negative impacts. It can mislead some young and inexperienced readers into thinking that abnormal relationships are acceptable. Moreover, it might face legal issues in some cases as it violates ethical and moral boundaries. Also, it will be shunned by the vast majority of the legitimate Harry Potter fandom who uphold the positive values of the series.
Another idea is that Harry, already being a hero, has to teach his sister all about the wizarding world. He shows her Hogwarts, but they face challenges like some students being jealous of her relation to Harry. They go on adventures in the Forbidden Forest together, and Harry imparts his wisdom and courage to her.
Hermione would still be a very popular character. She could be the one to help the twin sister study and understand the more complex aspects of magic. Her logical thinking and vast knowledge of magic would be invaluable in their adventures.
Their parents' old friends could be main characters. They might come to check on Harry and his sister, bringing gifts and stories about their parents. These friends could offer guidance and protection when needed.