Lord have mercy. I'm praying for you and for your husband! God bless you for taking care of him, and I wonder if you can get a break to take of yourself, recharge JUST a bit, to keep you strong? And I'm glad you have the support of your parents, pastor, and medical care.
Now, with that ...
I'm going to give you my honest impressions, but of course I may be wrong. Just something to think about.
I've known people who are so self-absorbed they are just oblivious to other people's needs or problems. Those kinds of people - well, if you give them a VERY direct statement (not a bad idea to write it out before you try to tell them) ... they may in rare cases wake up to reality. But I'm just saying this as a possibility. I'm not so sure that's what it sounds like.
If his parents abused him as a child, to the point that he is dealing with stuff like this now, it's possible the problems are deeper. I also know people who thrive on drama, who make everything about themselves, and who deliberately hurt others in the process. I don't know her heart (and maybe you don't either, I don't mean that in an offensive sense) but if she could say something like this ...
Then it seems like more than just being dense.
Bottom line, you are unlikely to change her. And also, you are dealing with a very stressful situation, and you need to take care of yourself and your family, not open yourself to being torn down so much that you fail yourself and your husband. (I actually doubt that would happen, but it COULD affect your health in the long term, or something like that.)
I've been in a somewhat similar situation. There were differences. But I did have to take a strong stance with someone who wanted to destroy my family, who might have some things in common with your mother-in-law.
I was polite, and calm, but firm. I told the person in a very matter of fact tone what the limits were. I told the person what I wouldn't accept from them, and then when they did it, I told them calmly that I must hang up if they insisted ... and then I did. A few calm hang-ups and they learned that was how things would go, and it got better.
That seems to be the best way to get the message across, and it prevents you from stirring up your feelings in a bad way. It may not solve the problems with the other person, but it takes you out of them.
And of course, because we are Christian, we are told that we must love them and forgive them. That doesn't mean you have to be subject to any abuse, or even like them. The best thing I have found is to pray for them, as well and as sincerely as you are able, and to pray to God to help you forgive. Try to see they may be acting out of hurt too. It takes time, but God will help your heart in the matter. And you can go on helping your husband.
I hope some part of this can help in some small way.
You have my prayers. God be with you and yours.