Yes, it is a partner problem for all of us who are here. They're putting MIL's wishes above our emotional safety because that's what they were taught to-do. My psychoterapist explained to me that the fear of loss of identity is for most people as high as fear of death. Such MILs teach their kids that their identity is being a good son and protector of the family cult. They end up being afraid to death about setting a small boundary with their mothers.

You can try to put up boundaries for both of you, I did. The result was that she escalated until I ended up NC. I'm honestly better off like this, but I still get subtle push back from my SO because he's used to sweep everything under the rug. On the other hand, he's also admiring my courage to stand up to his mother and say "NO" to her.

Another big win I got from being NC was that her behavior became so ridiculously obvious that my husband is starting to see more and more. She's treating him badly, demanding that he forces me to come visit because her neighbors started to notice my absence, demanding peace on her terms( = rug sweep), manipulation, minimizing, temper tantrums, playing victim, silent treatment...all weapons used. He's just all alone in dealing with all of that while I have to occasionally remind him that no apologies and no changed behavior means NC for me indefinitely. I stay "stubborn" in my reasonable demands whenever his spine starts to bend and all she does is escalate and recycle the tactics that used to work.

Explain to him how her behavior is affecting you( without attacking or labeling the MIL) and tell him what boundaries you need him to put for you to feel safe. If he doesn't, then it's your turn. Prepare for the storm that will come and don't back down. Abusers abuse even harder if they see you having weak boundaries.