Yeah I mean it's your dad's brother and all but they sound like trash... Don't feel bad for cutting them out on your end. Don't fall for any bullshit trying to make you feel sorry or responsible. Just don't acknowledge any attempt to make you the bad guy. Your cousin is an entitled bully and I would recommend you constantly refer to her as a bully to everyone who talks to you about her and/or the incident.

"Which cousin?"... "You mean the bully?"

She is literally about 4 years ahead of you in terms of development. It's the equivalent of you being punched by an 18 year old male. I'm pretty sure that wouldn't go down well with the police. In fact why don't you run it by the police. You were assaulted after all... If they won't see facts, then let the law make them see.