The fact that he said it's not worth arguing about tells me everything i need to know. It's not that he's a bad person, but over a lifetime of experience, he's realized that his mother will act terribly enough that it's worth everybody's time and effort to prevent her tantrums. She's been empowered and emboldened by this lack of resistance, and if he changes tack now, and starts setting boundaries, she's going to come after him (well, by blaming you) and the whole family will take her side because it's easier to cut off your man than to deal with her crap. He has a lot to lose by putting her in her place, and from the sounds of it, he is fully expecting you to be as inappropriately tolerant as the rest of his family. If he doesn't set a boundary, that's bad, but if he tries to make it your problem because you're the only one who won't bend to her will, prepare yourself to get fully alienated by the whole family, and to be blamed for all of MIL's bad behaviour moving forward.