My mother is 91 and in the middle stage of Alzheimers. She remembers very few people, not even her grandkids. She is the oldest of 4 sisters. Yesterday the last living sister besides Mom died of cancer. Should I tell Mom?
I wouldn't bother. She'll likely be upset. And ask about it over and over and you'll have to keep telling her and it will be new and upsetting every time.
So, nothing to be gained by telling her. Let her miss out on this pain.
That's a good way to put it: "Let her miss out on this pain." When you think about it, that may be the only "perk" to Alzheimer's: potentially never having to go through a certain loss for the first time, let alone again and again.
So, nothing to be gained by telling her. Let her miss out on this pain.