My elderly mother who has dementia, thinks my father has recently run off with another woman. She forgets that he passed away 35 years ago. At the beginning when this came up I told her the truth and she said I was wrong. After that, if it came up I would say I didn't know where he was and would try to distract her. That did not work and seems to made things worse. Right now she doesn't trust me. She thinks I know where he is and secretly visit him. She keeps asking more and more detailed questions about why he left her, what he told me, where he lives, do my adult siblings live with him etc. She wouldn't believe me if I said he was on a trip since he never really travelled. Looking for ideas on what to say to reduce her distress, stop the questions, and somehow rebuild trust.
Do you have any of the funeral cards/programs from your Dad's service? A photograph of your Dad's headstone - maybe after your mother decorated it with flowers some years in the past? Today many funeral homes will frame a nice picture of the deceased with a copy of the service program. Hanging something like this on the bedroom wall might help remind her of his death for a while.
I suspect only time (and maybe some anti-anxiety meds) will retire this line of questioning. Some day you may need to just count your blessings that she is obsessed over this topic and not something even harder to listen to like bowel habits or all the men that are coming to the house to see you (or the women your husband is seeing). When the brain is working badly it is amazing what detailed fantasizes it can create.
When my 96-year-old dad tells people I'm his dad, I go along with it. When he says his wife was just here and asks where she went, I tell him she went shopping and she'll be back soon (my mom died 8 years ago). When he says he wants to go home (to his childhood home where he last lived in 1940), I tell him it's too late tonight, so get a good nights sleep and we'll leave right after breakfast and his reply is almost always something like "Okay, that sounds good." I've found that accepting my dad's "realities" and using therapeutic fibs to make things smoother help keep him content and me less frustrated. And I use this same fib and divert technique with many of my dad's fellow residents in memory care, usually, but not always, with good results. Best wishes.