One idea could be that Harry's little sister discovers she has a unique magical power that no one else in the family has. Maybe she can communicate with magical creatures in a way that even Hagrid is impressed by.
Well, obviously the little sister herself is a very important character. Depending on how she's written, she could be really kind - like a Hufflepuff at heart - or really brave and adventurous like a Gryffindor. Then there are the classic characters from the original series that often play a big role. Hermione would likely be there to offer her wisdom and help the little sister with her studies. Ron would be the comical friend, always getting into some sort of trouble but also being very loyal.
The main characters are Harry, who is the focus as Sam and Dean's sister. Sam, who is good at research and understanding the deeper aspects of the supernatural. And Dean, who is more hands - on and ready to fight. Their different personalities and how they interact with Harry form the core of the story in this fanfiction.
The Weasley family would probably play a role too. Ginny could be a role model for the little sister, especially in terms of being a strong female witch. Fred and George might teach her some of their pranks or tricks. And Mrs. Weasley could be like a second mother to her, showing her the warmth of a family. The Malfoys could also be in the story, perhaps as antagonists, creating conflict for the little sister and Harry.
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.
Another theme is the little sister getting caught up in the world of fame through Harry. Maybe she starts getting followed by paparazzi just because she's related to him. It adds an interesting dynamic as she has to deal with this new aspect of her life.
One interesting fanfiction could be where Lucy discovers she has a unique magical ability that even Harry doesn't have. Maybe she can communicate with magical creatures on a deeper level. In this story, she uses her ability to solve a mystery in Hogwarts that has been baffling everyone. It could show her growing up in the shadow of her famous brother but still making her own mark in the wizarding world.
Hermione would be a great character to include. She could be like an older sister figure to Harry's little sister, helping her with her studies and giving her advice on how to handle the challenges at Hogwarts.
One possible plotline could be Harry going on hunts with Sam and Dean. She might use her own unique skills, perhaps some magic - like in the Harry Potter world - to help them out in a way that surprises the brothers. Another could be her trying to fit into their world of hunting while also dealing with the typical sibling rivalries and protectiveness.
One interesting plot could be the little sister having a unique magical ability that even Harry doesn't possess. For example, she might be able to communicate with magical creatures in a way no one else can. This could lead to her getting involved in all sorts of adventures and helping to solve problems in the wizarding world that others can't.