Normally I wouldn't respond, but I have to this time. My father had Alzheimers disease and died of it about 4 years ago. My mother had a bypass about two and a half years ago and never recovered from it . She died about two years ago. My older brother and his wife never spoke to them or any of the rest of the family for the past 10-12 years. And we never really knew why. On numerous occasions, I tried to contact my brother, but either he or his wife just hung up on me or returned my letters and photos. My 12 year old son doesn't even know his aunt, uncle and two cousins. None of my brother's family were at either funeral and never even indicated that they cared in the least. They couldn't find the time to come to my sister's husband's funeral either when he died at the age of 42. They sent a gift certificate to a grocery store about a month later to my sister.
Do not let this go on any longer. Try to find out what happened and forgive. Talk to a counselor. Ask the mom to go with your husband to a counselor and resolve this. At some point, your husband just will not forgive himself if he never talks to them again. If his father has Alzheimer's he needs to talk to him before he is forgotten. This disease is the most horrible thing.
Be the bigger ones. Give it up to God and forgive. Please.
This needs some prayer on your part.
PS. She can't say Alzheimers because she can't accept the fact that the man who took care of her, was the breadwinner, etc, is now being reduced to less than a child who needs help getting dressed, eating food and having to be watched 24/7. My father always tried to go outside and my mother was worn out trying to watch him all the time. She seldom got more than an hour or two of sleep because she would hear him get up and wander. She would lay out clothing for him and he would come out with his underwear over his pants. He had no idea what went on first or why. She yelled at him a lot because he just didn't get it. You need to find out more about Alzheimers because the biggest victim is the caregiver. They often need more medical help than the patient. Sometimes the patient get violent and will strike at or fight with the caregiver. Check for bruises on your MIL. She seriously needs your help.