Could you clarify something : does your mom actually KNOW about the physical abuse?

Have you actually said to her "mom, he beat her"? What about your sister?

Does she acknowledge the fact that abuse took place , or is she in denial?

As for the whole ' seeing dad sets mom off' drama...seriously? You and your sister are both adults. If you choose to see your father that is your business. Not her's. I get the sense that your mom has a low frustration tolerance and tantrums when she doesn't get her way. Even if her way hurts others.

If HER abusive nature keeps her from the grandkids, well, that is her reality.