Originally Posted by Rain Man
Let's do a scenario analysis. Others can chime in if I'm missing something.
Scenario 1. You go and he goes. You both want to be with your mother on the holiday. You both have equal access, and it shows that he can't control her access. It's a draw until she decides that he has to go.
Scenario 2. You don't go and he goes. It shows him that he can further control your mother just by showing up. He limits her interaction with you, and he wins. I don't think it'll force a long-term breakup, so there's no direct win here other than maybe a short-term win by creating Scenario 3.
Scenario 3. You go and he doesn't go. If he doesn't go at your mother's request, it may put her more at risk for abuse later, though I think it's a long-term win. However, it probably causes her some short-term resentment toward you and won't do anything to solve the long-term problem. It's a win for you, but with some damage.
Scenario 4. You don't go and he doesn't go. Your mom eats a bunch of turkey and falls asleep on the couch during the Lions game.
My MIL is an absolute bitch of epic proportions. My wife and I have nothing to do with her, and her only contact with our oldest daughter has been mediated by the courts (she has no contact with my youngest daughter). My wife's step-father's family disowned him (step father is actually a pretty good guy) when he entered into a relationship with my MIL. Disowning him only drew him closer to my MIL.
If you don't go to support your mother, you are only pushing her closer to the bastard. She will feel as though she has lost her family and will turn to the one person she feels she has a close relationship with--no matter how bad and destructive that relationship is.
Maybe you could invite her (without her significant other) and your brother's family to your place. I know it is inconvenient for you, but it is a minor inconvenience compared to the importance of your relationship with your mother.
Have you explained to her that you don't feel comfortable having your children (her grandchildren) around the guy?