Perhaps she's a sister who was thought to be dead but actually survived. She could have been corrupted by the experiences she had while being 'dead'. When she returns, she blames Percy for not saving her properly. This false blame leads her to betray Percy. She could start spreading rumors about Percy being a coward or a bad leader among the demigod community.
Percy's ability to communicate with sea creatures could prove useful in Hermione's world too. He could ask the mermaids in the Hogwarts lake for information, and Hermione could use her magic to translate if there are any language barriers. And on the other hand, Hermione's magic could help Percy when he's in a situation where his demigod powers alone aren't enough. For example, she could use a spell to make him invisible when he needs to sneak around in a place where his powers might be detected.
One idea could be that they discover their relation during a magical crisis. Maybe a dark force is threatening both the wizarding world and the demigod world, and their combined powers as siblings help save the day. For example, Hermione's magic and knowledge of spells combined with Percy's control over water and his demigod strength.
In fanfiction, anything is possible. Writers might create a crossover universe where the magic systems of the 'Harry Potter' world (where Hermione is from) and the 'Percy Jackson' world merge. Maybe there's some kind of ancient prophecy or magical event that links their families. For example, a powerful god or wizard could have meddled in the past, making Hermione and Percy siblings in this new fan - created narrative.
Maybe they'd have to save their family from a common enemy. Since they are siblings in this fanfiction, their family could be in danger from a being that wants to use their combined powers. This would lead them on an adventure through both the wizarding world and the world of Greek gods, having to use Hermione's knowledge of spells and Percy's ability to navigate the world of the gods to keep their family safe.
Yes, there might be. Fanfiction is a vast world where all kinds of crossovers and creative ideas are explored. You can start by looking on popular fanfiction websites like Archive of Our Own or FanFiction.net. Just search for the specific concept and see if any authors have written about it.
Well, one possibility could be that the sister discovers she has some unique powers that are different from Percy's. For example, she might be able to communicate with mythical creatures in a way that no one else can. This could lead to her getting involved in a quest to save a particular part of the mythical world that is in danger.
A common theme is the discovery of the sister's powers. For example, she might find out she can control water like Percy but in a different way. This adds an interesting twist to the story as they learn to use their powers together.
I really like 'A Sister's Tale in the Percy Jackson Universe'. In this fanfiction, the sister character brings a new dynamic to the story. It shows how she copes with the knowledge of the Greek gods and her relationship with Percy, which is full of both love and typical sibling spats. It's a very engaging read.
One great 'Percy Jackson and Sister' fanfiction is 'The Lost Sister'. It has an interesting plot where the sister has unique powers and they go on adventures together. The character development is really well - done.