One possible plot could be that Lemon and Harry discover some long - hidden family secrets together. Maybe they find out about a special power that runs in their family that Voldemort is also interested in. So they have to race against time to master this power and keep it out of Voldemort's hands.
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.
One possible plot could be that Harry and Lemon discover they have special magical powers that are unique to their twin bond. They might go on a quest to find out more about their family's hidden magical history. For example, they could find an old diary that leads them to a secret chamber in their family's ancestral home filled with ancient spells and potions.
One possible interesting plot could be their adventures at Hogwarts together. Since they are twins, they might have a special bond that helps them face challenges like the Triwizard Tournament. They could work as a team to solve the mysteries surrounding it, using their combined magical skills and the unique connection they have as twins.
Well, it could be about their adventures together in the Hidden Leaf Village. Maybe they train together under Kakashi, and face challenges that test their bond as siblings. They might also have to deal with the fact that Lemon has some unique abilities that set her apart from Naruto.
Well, perhaps the twin could have a special power that helps Harry in his adventures at Hogwarts. Maybe they work together to solve mysteries within Griffendor, like finding a hidden chamber or uncovering a plot against the house.
It could also be that Harry and Lemon inherit an old family heirloom that has some sort of magical power. But they don't know how to control it at first. So they embark on a journey to find out more about it, meeting various interesting characters along the way, and in the end, they learn to master the power and use it for good.
One common theme is family. Since she's Harry's twin sister, the stories often explore their family connection and how it affects their lives in the wizarding world. Another theme is identity. The sister has to figure out who she is in relation to Harry and also as an individual witch. Magic is also a big theme, of course, like how her magic might be different or similar to Harry's.
Another way their magical abilities could differ is in the area of spell - casting. Harry is good at defensive spells, but his sister could be excellent at transfiguration. This would mean that in different situations, they would rely on each other's skills. For example, when they need to transform something to solve a problem, his sister would take the lead, and when they are under attack, Harry would be the one to defend them using his defensive magic.
Ron and Hermione are likely to interact with them. Ron might be a bit more protective of the sister, while Hermione would probably study and share knowledge with her. They could all be in the Gryffindor common room, discussing spells and strategies.
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.