I assume this thread was inspired in part by my Christmas post.
My husband made it very clear that my MIL is certifiable even though she initially comes off as pleasant. We only have to see her the one time of year and I can live with that. Does it ruin Christmas? Yes, but that's ok. I can deal with that for my husband. He has his own way of coping with her.
If anything, I am mostly disappointed that I haven't figured out how to deal with her. Usually I am very good at handling difficult people on a short term basis but this is a challenge I haven't conquered yet.