Nephew and his girlfriend

My nephew has been going out with girl for just over six months and he recently moved in with me and since he has moved  in I've seen that she is really controlling and she plays with his mind his GCSES are next week and he doesn't need her playing with his head. I can't have a conversation with my nephew with out her hearing it because she is always ringing him and won't leave him alone for 5 minutes. I am lucky if I get to see him for 10 minutes apart from when he comes and eats dinner with me. Me and my nephew have been on holiday recently to see his mum and she asked if she could come with us bearing in mind this is the first time I have seen my sister ing two months I told her no then she asked if she can meet us and have a day with my sister and her other kids I said no, she then began being horrible to my nephew because I said no I had to take his phone off him so he would spend time with his mum and sisters.