Can you prevent your child's grandmother from seeing him/her if she is unstable?

I was just wondering. I am not pregnant nor do i have any children but my bf's mother is a very emotionally unstable person. i'm not the most stable person out there but more than her. She's gone through a lot of emotional and physical trauma which caused her to drink excessively to the point of not being able to walk very well without stumbling. (this has happened a few times when we hung out) She has a slew of health problems and even though she's watched my bf's niece before and things were ok (the child wasnt hurt but things were broken) i dont fully trust her watching my child if i ever do have one with him. I'm wondering if there is some law out there that forces the mother to let the grandparents see the child if the mother doesnt feel the grandparents are mentally capable or approve of how they do things.

Normally no. The parents decide.

A few states allow grandparents to ask for visitation in court but at that point if you outline your concerns the courts normally respect the parents as the primary decision-maker. And they won't force visitation.