How should I deal with my MIL who is so self-centered that her husband's dementia is a huge burden to her?

If MIL has always been a self-centered drama queen, as ps says don't expect her to give up the crown any time soon.

Probably the best you can hope for (and work toward) is that she does not interfere with your decisions about what is best for FIL. Someone has to be responsible for his care, and that sounds like it is up to you and your husband.

In her defense (a little bit) it IS terrible what her life has become. She is in a state of mourning for her losses, and her losses are great. Of course, that is true for their son and for you, too. Be grateful that you two are able to cope with this much better that she is. She is to be pitied.

No matter what "should" be happening, it doesn't sound like you can count on her much for FIL's care. Supplement your own efforts with in-home care as long as you are comfortable keeping him at home with you. Be aware that in most cases, dementia progresses to a point where home care is just not feasible.

Unfortunately my Mom was that way with my Dad before he passed from cancer. She did not want to take care of him and put him in a nursing home. He then promptly died. Now she lives with us and is a very grump awful person who is overly demanding. I wish I did know what to say to you, other than to let you know you're not alone. Older people can be really mean.

Don't expect the drama queen to give up her crown any time soon. It might help if you can get her a prescription for Ativan, she can take it when she is overwhelmed. At the same time, talk to the MD about facilities for Alzheimer's nearby, because once he loses speech and starts wandering, he will need 24/7 care. At that point mom will do extensive grieving.