I agree with Cicero. Definitely opt out of the gift exchange and explain why to your son. Your mother will miss out on her audience and opportunity to hurt you, your son will not be exposed to it, and you won't get another pink bathrobe. If your brother asks why you're not there, tell him exactly why. It seems he understood after the picture incident any way. Maybe he will rock the boat again, maybe not. But your family will be spared.
You are not alone. There are many of us "survivors" here. My family cut back severely on interaction with MIL long ago and our kids are the ONLY grandchildren. This year MIL fawned over her tenant's child.
(She has no one left!!!) She showed us pictures, told us about the gifts she gave them, fawned over the gift the tenants gave her. Did not open ours. It was so over the top crazy, her SIL who is very reserved, told her to knock it off. My daughters, teenagers, looked at me and chuckled. We explained the dynamic to them long ago. They grew in the knowledge of it, don't miss the "interaction," and embrace the distance. They know that no gift in the world is worth all that pain.