Well, perhaps she grows up in a different environment than Harry, away from the Dursleys. This different upbringing shapes her character in a unique way. She might have a natural talent for a particular type of magic that Harry doesn't. For example, she could be really good at divination and use her skills to foresee the coming dangers to Harry and the wizarding world. And she would be constantly trying to keep her identity as Harry's sister a secret from the wrong people while still trying to support him in his fight against evil.
In this fanfiction, the sister could be born with a prophecy attached to her as well, just like Harry. But instead of being the 'Chosen One' in the same sense, she is the protector of the Chosen One. She might study ancient magics that have been long forgotten in order to gain the power to shield Harry from Voldemort's attacks. She would also have to deal with the jealousy and suspicion of some in the wizarding world who think she might be using Harry for her own gain, when in fact all she wants is to keep her brother safe and help him defeat the dark side once and for all.
Well, it could be that the sister was kidnapped by dark wizards when she was a baby. Years later, Harry discovers some mysterious clues that lead him to believe she might still be alive. He then embarks on a dangerous journey to find her, facing many magical obstacles along the way.
One possible plot could be that Harry discovers her relation to Sam and Dean early on. She then tries to balance her wizarding education at Hogwarts with the hunting life of her brothers. Maybe she uses her magic to fight off the same demons that Sam and Dean are hunting, but in a different, more magical way.
One possible plot is that Ichigo is reborn as Harry Potter's long - lost twin. He would then have to deal with the same kind of prophecy and the threat of Voldemort. But being Ichigo, he would approach things in his own way. He might not rely so much on magic wands but use his own spiritual power which could be disguised as a new form of magic in Hogwarts. This would cause a stir among the professors and students alike.
Well, perhaps Harry, reborn as Frieza, would use his wizarding knowledge in a different way. He could combine magic with Frieza's powers. Maybe he'd use spells to enhance his energy blasts or use his knowledge of potions to create new substances that could boost his strength even more in his new Frieza form.
Well, perhaps he would grow up with the unique Dorne culture. He might be trained in their style of combat which is different from what he learned in Hogwarts. And he could use the Dornish knowledge of poisons and healing in his new adventures.
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 possible plot could be that Lemon discovers she has a special magical ability that is different from Harry's. For example, she might be able to communicate with magical creatures in a unique way. Harry then has to protect her from those who might want to use her ability for bad purposes.
Well, it could be that Sakura, with her knowledge from her previous life, might bring some unique magic from her own world into the Harry Potter universe. Maybe she becomes a student at Hogwarts and uses her special skills to outwit the dark forces there, like Voldemort. She could also form new friendships with the main characters, and her different perspective on magic might change the way things happen in the story.
Well, if Harry Potter is reborn as Percy Jackson, perhaps his relationship with the gods would be unique. Since he has experience with a different kind of magic system, he might question the Greek gods' ways more. He could try to introduce some of the more organized aspects of the wizarding world, like the Ministry of Magic's rule - making, to the rather chaotic world of the demigods.
Well, perhaps Harry could be reborn as a shinobi. He might use his magical knowledge in unique ways in the Naruto world. For example, he could use magic to enhance his jutsu or come up with entirely new combat techniques by combining magic and chakra. His Hogwarts education in spells and potions could give him an edge in the shinobi world, like using potions to heal more effectively than regular medical ninjutsu.