Here are the specifics of my mom not remembering that my dad died: She might first ask why his ashes are on her dresser. I explain to her that I put them there because she forgets that he died, so I'm thinking the ashes will help her remember. Then she asks what he died of. I explain he had a fall and was in pain for a year and then took his own life because he was not getting better. Or she asks where he is and I tell her he died and then the entire story again. After that she offers her memories about what a good man he was, how handsome, and kind he always was, and her imaginary story that the room she is in is where they first met. I don't correct her about the room, but totally and truthfully agree he was wonderful. This has been going on for months, and she has seemed to appreciate the truth, but now suddenly it seems to upset her. The care givers might tell her that he is in heaven, but she does not believe in heaven, so that confuses her and she asks when he is coming back. Do you think I should stop telling her how he died? I think she can deal with him having died, but his suicide is what is upsetting. What would you say? I'm not good at lies.
Thus, regarding your Dad, time to remove the suicide from the conversation. Tell Mom that he had a serious fall and passed away from that fall. If Mom accepts that, then use that information each time she asks.
You will find later down the road when Mom doesn't believe your Dad had passed, then you can use therapeutic lies to help Mom once again. Example, tell her Dad is visiting friends and should be home next week. By tomorrow, Mom may forget that, and ask again.
Let us know how that works.