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.
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.
One possible plot could be that his grandfather, perhaps a powerful but reclusive wizard, trains Harry in unique magic from a young age. Harry might grow up with a different set of values and skills compared to the original story. He could have a closer bond with the wizarding world's older traditions.
In fanfiction where Harry Potter is raised by his fans, the plot could involve a lot of interesting scenarios. For example, he might be exposed to a much more modern and diverse view of the wizarding world from the start. Fans could introduce him to different magical cultures that were not fully explored in the original books. He might also grow up with a stronger sense of self - worth as fans would likely shower him with love and support, which could make him more confident in his abilities.
The plot would change drastically. Harry would not be living with the Dursleys, so there would be no need for him to escape that horrible situation. Voldemort might have a harder time targeting him as his parents would be on high alert. There would be no letter - hunting drama. Also, his relationship with Snape would be different as his father was Snape's rival. His parents could have taught him some advanced magic from a young age, which would make him stand out at Hogwarts in a different way than in the original story.
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.
It would be a very different relationship. He might see Hogwarts as a foreign place at first. He'd be used to Odin's teachings, which are very different from Hogwarts' curriculum.