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.
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.
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.
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.
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.
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.
Perhaps in a fanfiction where there is a supernatural force at play. This force makes Sam act out of character and he kisses Dean on the neck. Dean is initially shocked but then realizes that Sam is not in control. They then have to figure out how to break the spell while also dealing with the new - found awkwardness and the underlying emotions that the kiss has brought to the surface.
It could be that Sam found out some dark secret about Dean that he couldn't overlook. For instance, if he discovered that Dean had made a deal with a really evil force in the past to save Sam's life, but in doing so,Dean had unknowingly put many others in danger. Sam, with his strong sense of justice, might feel betrayed himself first and then in turn betray Dean to set things right.
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.