TW: childhood SA, childhood abuse

So. I (21F) was extremely abused in my childhood, but one thing in particular was that I was molested from the ages of 5/6-8 years old. When it came out, my mom took my abusers side (her husband) and abandoned me.
My grandmother was encouraging my mothers behavior and tried to psychologically torture me to get me to change my “story”. They both are horrible gas lighters and extremely passive aggressive. 
I haven’t been able to sever ties with them, but I want to.
My grandmother wants to meet with me, how do I tell her no?

TL;DR My abusive family wants to meet with me, how do I tell them no?