Should I watch my niece?

Because my mom doesn't want her at my sister's (the child's mom) because she always gets babysitters on the weekends to go out and drink? Really, my sister isn't calling me, my mom is. My mom has her all the time. My mom is taking my youngest sister to the Capitol One Bowl this weekend bc she's in the halftime show so she won't be home. I love my niece, but I don't like having to watch her when her mother is perfectly capable and just won't do it. She hasn't got a bond with her daughter I think. I know it's bad to say but she was 17 when she had her and she was born really early at 27 weeks and stayed in the hospital a long time. But at any rate, she is still trying to be a kid and go to the clubs every weekend. What should I tell my sister? I am going to talk to her, but I want her to listen so I can't offend her. Even though, if I said what I thought I am sure that I would.