OP, you have my sympathy for your loss, and for what you have to go through now.
My mother passed away 3 years ago, and I had some issues dealing with the inlaws after her passing. First, DH had a hermit uncle (yes, most definitely some psychological problems there). For some reason, there were several relatives who did the grocery shopping for him, carted him around (when they could get him out of the house; he was a lifelong bachelor so had no wife or kids of his own), etc. Basically, instead of getting him help or encouraging him to get help, they enabled him. For years. He got more and more and more reclusive. (This man had been very active and outgoing years before, so this reclusiveness was a somewhat new thing; I have my own thoughts on that.) He refused to go to the doctor, even when it was obvious there were problems. And when he finally did go, it was too late because he had cancer and a host of other problems.
My mother had a chronic condition which could be managed, but she also had a weaker heart than average. The condition took a toll on her heart. My sister and I did everything we could to manage her condition and keep her with us as long as possible. She knew this (well, some of it), and was very appreciative, and trusted us to make the best decisions for her, even if she didn't want to do it (no chocolate? blasphemy!
). So after my mother passed away (and I was right there with her), I couldn't stand to hear ANYTHING about DH's uncle. We tried so hard to keep my mother alive, and when they talked about him, all I could see was a man who refused medical help until it was too late to have any benefit. Whenever my MIL would bring him up, I'd grit my teeth and say nothing. Finally, within a few months after my mother died, DH mentioned something about the uncle or seeing the uncle since he was sick, and I lost it. I told him, tearily and noticably upset, that I just lost my mother, and if there had been some way keeping her alive longer, some other medical thing, we would have done it. Without hesitation. Not a question...we would have done it. Yet there was nothing that could be done for her anymore. Yet his uncle could likely have had a much better prognosis, yet he REFUSED medical treatment, or even to SEE a medical professional, for months (could be years...I don't know). He had it available to him, and he REFUSED it. So, I told him, don't talk to me about going to pander to a man who values his life so little that he could essentially throw it away, when my mother just wanted to live and went through a lot, weekly, just so she COULD continue to live.
It was strong, I know. There were circumstances of uncle's mental state, I know. It wasn't as cut and dried as that, I know. But that didn't matter to me at the time. I had just lost my mother, and I told DH that I could just NOT handle seeing or hearing about the uncle. It was like sticking a knife in my grief and twisting it.
OP, I don't know how your DH is, but maybe you need to get a little bit mad and emotional for him to understand. Contrast what you were going through, with what MIL said at a certain time. "DH, I was in the hospital, knowing that I had hours left with my mother. EVER. And your mother said XYZ. Do you know how that made me feel? Do you realize that I will ALWAYS remember that she chose to do/say that?" Tell him that the wounds are still very fresh, and you still remember that like it was yesterday. So to be around her at Christmas, which meant something special to your mother but was just almost an ordinary day to his mother? No, you do not need to be around MIL on that day.
Perhaps 'she didn't mean it'. Fine. But you're feeling what you're feeling, and you have the right to feel that way. Your MIL is a big girl and she didn't just lose someone. In fact, she told you the day had no special meaning. So you're taking her at her word. Tell him you need him to have your back on this, even though he may not understand it completely. And if MIL gets mad? Tough. She can get over it or die mad (to use a phrase I've seen on this board numerous times).
Hang in there. And again, you have my sympathies.