Problems with my negative grandma?

I am helping my grandma this summer and am having trouble dealing with her constant negativity. My parents are getting divorced and now all my grandma does is criticize my mom. She goes on long rants about everything she thinks is bad about my mom and I have to just sit there and take it. My dad is unemployed and has an awful temper and constantly yells at us for the stupidest things and we've had to live with that for years. I want to explain to my grandma that what she says hurts me and not everything is my mom's fault, but she so argumentative I don't feel I could get through to her. Plus she's supporting me a lot financially and I'm afraid to upset her because I do depend on her for a lot. How should I handle this?