I'm going to get my cousin grounded
Why you would do that to your cousin?
He has a lot of problems and causes problems at home.
It sounds like your cousin is going through a rough time. Rather than getting him grounded for his actions, why not spend some time with him to help him hash out some of the issues.
Who are you anyway? How the fuck do you make the decisions for him?
I understand your frustrations about your cousin. But there may be something going on with them that he hasn't made known that makes him act out this way. I would recommend you talk with her cousin to see if there's anything going on with them.