The most important thing to remember is that sometimes you will have a family member who deserves to be treated no different than a stranger. Abusive people rely on bonds to continue the abuse. Sometimes cutting ties in some form is best.

If you're getting dragged in through your mother, and you have good a relationship with her, you could just say that you're not getting involved because of how it affects you. Grandma will be dead soon anyway, and a lot of things change when matriarchs pass.