That is horrible. It doesn't sound like you can trust your grandmother if she thinks abuse is normal and was brought up that way.

When people are brought up in a toxic environment and it's all they have known, it's very hard to change the person's mind.

I know it hurts that she seems to accept him being abusive is okay. No, it's not okay. Did you let her know that was not OK? Do you have to keep Interacting with her?