Yes, there might be. The 'Supernatural' world of Sam and Dean and the 'Harry Potter' universe are both very popular, and fanfiction often likes to cross over different fandoms in creative ways. Some fans might have come up with the idea of making Harry Potter be related to Sam and Dean as their sister in a fan - created story.
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.
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.
The little sister herself can be a great character. She often brings a fresh perspective to Sam and Dean's world. She might be brave but also have a vulnerability that makes her relatable. For example, she could be hesitant about the hunting life at first but then grow into a strong fighter.
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 interesting 'Sam and Dean little sister fanfiction' story could be where she has special powers that she has to keep hidden from the supernatural world while still being part of Sam and Dean's hunting adventures. For example, she might be able to communicate with spirits in a different way than her brothers, which adds a new dimension to their investigations.
One common theme is family protection. The little sister is often in need of protection from the various supernatural threats, and Sam and Dean are always there to save her. For example, in some fics, she might be targeted by demons because of her connection to the Winchesters.
Fanfiction.net is also a great place. There are many dedicated Supernatural fans who write stories about the Winchester family. However, the quality can vary, so read the reviews or summaries first. Some writers might focus on the relationship between the brothers and their little sister, while others could explore her own adventures in the Supernatural world.
One common theme is protection. Sam and Dean are always protective of Harry, and Harry also tries to protect them with his magic. There's also the theme of discovery, as Harry discovers the world of hunting and Sam and Dean discover the magical world through Harry. It makes for an interesting exploration of different powers and cultures within the fanfiction.
Another good one could be 'When Harry Joined the Winchesters'. In this fanfiction, Harry's presence changes the way Sam and Dean operate. His unique skills and his relationship with the two brothers are explored in detail. It's a long - form fanfiction that really dives deep into the concept of Harry being part of their family.