My mother is obsessed

In just a few short days, I will be going to visit my very ill grandfather, because I would like to see him and I want my him to meet my son. My son is his first great-grandson. The issue is I don't want to visit my mother. Some background info, my parents divorced when I was 10, and I had to go with my mom. When she got there, she started hooking up with my grandparent's neighbor's son. I immediately didn't like him because every time I met him, he was higher than a kite. My mother decided to move in with him about a week later. Soon after, there was a lot of abuse. My mother stayed with him anyway, so I got my dad to come get me. In the entire time I was with my father, she hadn't bothered calling me (she would call my dad though, begging for money). Once I became pregnant though, so many years later, and she found out, I began getting calls non stop from her, trying to pry herself into my life. She would only ever talk about him, how much she loved him and missed him, ect. She would never refer him by his name. At first she called him "my grandson", which isn't so bad, but then began calling him "my baby". She continues to talk about him like he's her baby, and will never talk to me about anything other than him. When I told her I wouldn't be staying at my grandparents house (she lives with them) she went crazy, trying to get us to stay there. She lied about not smoking and their being five pets in there house. My problem is I don't want her around my son, I feel that she deserves nothing from me (she thinks she is entitled to see him and be in his life). But I don't think that's fair to my son, he has a right to know her, even if I don't want him to. How can I keep my cool when I'm visiting? I don't want to make things harder on my grandparents, but I feel like my mother is going to make me blow up.