I take care of my 79 (80 in July) mother in law and last week she told my husband you don't know how she is when your not here insinuating that I am mean to her (I am not).
I clean for her, cook for her whatever she wants on the days I'm here. I'm here at least 3 days a week.
But, I guess you have to try to put that aside, assuming this is caused by dementia and do your best to ignore this and not take it personally.