I want my mum to cut off contact from my aunt and cousins
That sounds a bit extreme. Why do you want her to cut contact from them?
She think my aunt is a mafia type and she visits me too much.
It's wrong to tell your parents who to cut ties with. You should let her do what she wants, she is an adult.
No, I'm going to tell her what to do and she has to listen to me.
Usually people do not ask their families to cut ties with other family. This is an extreme proposition to make.