Since your dad and sister seem to be coping well enough with it, I suppose my question is, why are you heartbroken? She is not betraying your father; honestly, she probably thinks this man IS your dad, even though she recognizes your dad when he comes to visit. I once had a resident who actually mistook another woman for her husband and got mad when anyone tried to separate them. Your mom is doing the best she can with the disease that she has, and if this man makes her happy (whoever he is to her) then that is great news! Even if it’s uncomfortable! My only concern would be if she is able to consent to intimacy, and how far along this man is in his dementia, as well. That’s the one thing I’d keep an eye on. I actually have a whole chapter about relationships and sexual health in dementia care in my book.