She could be a nephilim, half - angel and half - human. This would put her in great danger as both angels and demons would be after her. Sam and Dean would have to constantly protect her and figure out a way to keep her powers in check.
There's a fanfic called 'Sister's Bond'. In this story, the sister is brought into their lives in a rather mysterious way. It shows how she struggles to adapt to the life of hunting monsters while also trying to understand her new - found brothers. It's full of action and some really heartfelt moments between the siblings.
In a fanfiction, their sister could be a prophet. She starts having visions that lead the boys to new cases. But there are forces that want to use her for their own gain. Sam and Dean would have to fight not only the regular demons and ghosts but also those who are after their sister.
Obviously, Dean and Sam are main characters. They are the protective older brothers. And then there's the baby sister, who is likely a central figure as well, with her own personality and role in the fanfiction. There might also be other recurring characters from the 'Supernatural' universe like Castiel who could play important parts in the story.
The main reason is that it adds a new dynamic to the existing relationship between Sam and Dean. A sister character can bring in a whole new set of emotions and storylines. For example, it could be about protecting their sister, or the sister protecting them in a different way than they are used to.
One interesting storyline could be the baby sister having special powers that she has to learn to control. Maybe she can communicate with spirits in a unique way, different from Sam and Dean, which adds a new dynamic to their hunting adventures.
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.
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.
One great fanfiction could be where Dean and Sam have to face a unique Halloween - themed monster. Maybe a witch who uses Halloween traditions to cast powerful spells. They would have to use their wits and hunting skills to stop her before she causes chaos in a small town.
There's also 'A Hunter's Legacy'. It's set in an alternate universe where Dean and Sam face different challenges. The author does an amazing job of capturing their personalities. The story is full of twists and turns that keep you on the edge of your seat, and it really showcases the unique bond between the two brothers.
A unique plot could be that their marriage gives them new powers. Since they are so closely bonded now as husbands, they can combine their powers in ways they couldn't before. For example, when facing a powerful demon, their united power makes them invincible.