My mom-in-law, 84, was always an exceptionally wonderful person. Since my father-in-law, 8, had heart issues which are now resolved, she has started obsessing over little issues with family members and is completely alienated herself. She believes her brother-in-law lied about moving a brick or piece of tar between their yards. There is no rationalizing with her. This was always a close, fun, fantastic, loving family but now she can't even give her condolences to her brother-in-law for losing his beloved 37 year old daughter. She's said he, her sister and othrrs use the loss. My husband feels so hurt by her behavior and needs someone to give him some help. My in-laws live outside Pittsburgh unfortunately and won't come here and won't agree to getting family counselling.
Maybe print this out for the in-laws and share your concern in the most loving way you can! This is tough stuff, whatever the diagnosis or diagnoses. If they would come in to Pgh just for a day the Benedum center there does a wonderful job with comprehensive geriatric evals. They helped me and my mom a great deal, as did the neurology folks in the next building that I was able to arrange to see us the same day.
There is also the human tendency to obsess and focus on little inconsequential things when a great big thing that can't be fixed is looming that could be a factor.
Your job now is research, and being a support to your husband. Tolerating all these worries and fears is going to be hard. Best wishes to you all.
Your poor FIL is the one on the scene who needs to get things moving. Well now. His loving, beloved wife has recently supported him through a major health event; if there was a silly dispute of some sort he may feel protective of her about it; if they've both rejected family mediation it's not a good sign that he's willing to go against your MIL's feelings to seek help. This is going to be hard on everyone but especially hard on him.
What you can do is get the research done, get the literature together, do the homework on comparing options, and be ready to support your husband in supporting his father to help his mother. Other than that, I really wouldn't jump the gun - you could make things worse with the best of intentions. It's horrible, but you'll have to await developments.
And meanwhile, keep relations with the rest of the family as cordial as possible without undermining your parents-in-law or breaking confidence - a tricky diplomatic path to tread, but worth the effort.
If she has dementia, she will have no insight into her behavior. She will never believe that she has dementia. She will never apologize, because she cannot see - CANNOT see - that she has done anything wrong.
This information is hard to accept, because she is still intelligent, and in some ways, seems like the same person. But she isn't.
If someone can get her to the doctor, send a letter first so that you will not have to tell about her changed behavior in front of her. God bless you. You have a hard road ahead. Love your husband, and let him rant if he needs to. It must hurt a lot.
The other advantage of having a diagnosis (besides getting a treatment plan) is that family members will be better able to accept her. If her grieving BIL knows she has dementia (for example) he will not take her lack of compassion so personally.
There are many posts on this site about getting a reluctant parent to a doctor. If you need help with that, start another post and you'll get lots of suggestions.
This situation is very sad, and unfortunately is not unique. Many families face issues like this when a loved one suddenly or gradually develops behavioral problems. I wish you all a time of understanding and healing.