Is this legal or should I talk to her supervisors about it?

I don't know if that is illegal or not but it is DEFINITELY crossing the line. You need to talk to your boss about that and tell her that she isn't to change, decide, or mess about with personal things like that. You also need to address your sister and tell her everything. They think they are doing the right thing, and it sounds like you haven't told them that YOUR input is the most important. You NEED to do that.