I’m sorry your sister is getting bullied. I’m not sure if they had a friendship enough to where your sister can ask her why she’s being like this, and that her behavior is hurtful.

Or if this is one of those cases where the bully needs a punch in the face to respect boundaries.

That also made me wonder if the bully is getting bullied herself at home or by other kids, so she’s acting out because she’s miserable too.