Hi SmartCat,
Your in-laws are lucky to have someone so caring and concerned for them.
My uncle had Alzheimer's and, although my aunt was never in denial about it, she absolutely refused help of any kind until just a couple of months before he died. My mother and I offered every single week to stay with my uncle so my aunt could go out and do some things on her own. (The four of us did go out for breakfast every Sunday.) She did get her hair done one a week, but would leave him home alone despite our offers, which wasn't a good thing.
When we broached the subject of assisted living or getting someone in to help, they both were adamantly against it. My aunt thought that it would be a betrayal of him not to do everything herself. Her own health suffered greatly for it.
What finally turned things around was getting her in touch with a case manager who specialized in Alzheimer's patients. She talked to my aunt and told her that what often happens is that the caregiver will refuse help, become ill and end up in the hospital, leaving the Alzheimer's patient home, and everyone scrambling to figure out what to do.
She said that 100% of families she's dealt with thought it would be cruel to put their Alzheimer's patient in any kind of facility, but almost all of the patients actually improved and thrived once they were over the transition to the new location. She had an answer for every argument my aunt could come up with. After that, she oversaw every aspect of my uncle's care and was a great source of support for my aunt.
My uncle was quite upset at first at the prospect of strangers coming in to help him, but after the first visit, he just loved it when the nurses and aides came over. He liked talking to them about things that had happened in his life, and thoroughly enjoyed the attention. It was such a help to my aunt and gave her a little breathing room.
That's a long route to my suggestion, which is to get in touch with your local Alzheimer's Association and explain to them what's going on with your mother-in-law. They probably know of someone like my uncle's case manager who could talk to your mother-in-law and address her fears and concerns. I think that, a lot of times, it's easier for people like your mother-in-law to talk about things like this with someone outside the family.
Another possibility is to enlist the help of your father-in-law's family doctor, or whoever diagnosed him. Maybe you could ask the doctor to call your mother on the pretext of following up on the recommendations given at the time of his diagnosis, and he or she can impress on her how important it is for him to get treatment and the appropriate medication. Does she know that Alzheimer's medications can slow the progression of the disease for a lot of people?
I can't think of any other suggestions offhand, but maybe someone else can offer some ideas. I hope you're able to get through to your mother-in-law. I know how frustrating it is to see her in such a tough situation and refusing the help she needs.
Best wishes.
Carol