My sister is often misunderstood by our parents, but I don't feel like sticking up for her.
Misunderstood in what way?
They only care about her grades, they don't care about her as a person.
Why not bring this up to them? It sounds like an issue that needs to be addressed.
It's not my responsibility. They don't act that way with me, so I really don't care.
It may not be your responsibility, but don't you want to help your sister?