Me, too
Hi, Pear trees, sorry to say I do know what you're talking about, although not to that extreme.
In our family's case my mother cut her mother out of some photos of both her parents together. Given that my mother and her mother never got along and had a horrible relationship, this was not a surprise.
What was a surprise was when I was cleaning out my mother's place after her move to the care home. I found my mother's papers and photos (in odd places) and some papers and photos relating to my mother's father and his family (although a bunch were missing), but NOT ONE photo of my grandmother and her family. Not a single one. I know there were some, as I'd seem them when I was a child, but nothing turned up. I don't know if my mother got rid of them after my grandmother went to the care home or died, or if my grandmother (who also had Alzheimer's) got rid of them herself.
Now at the care home, we have given my mother a pile of her most favorite framed photos (although I was careful not to take any of her mother!) and I have noticed they move around a lot. Different family members get set on the bureau depending on how she is feeling about them, perhaps. The other ones are in a drawer or cupboard (fewer hiding places at the care home, thank goodness), but whether they are being kept our of sight, or kept "safe," I couldn't tell you. It's hard to know.
I'm sure it all has something to do with the dementia but couldn't tell you what. I'm sorry for anyone who has lost family photos or documents this way.
(And Bigreader, not to hijack Pear trees' thread, but I'm sorry your mother would say such a hurtful thing to you.)