Should I explain to My Broter in Law who has alzhiemers that his wife has cancer and is dying?
This is a tough question! Sorry that you are in such a tough spot.
I have had experience telling many of my demented patients bad news regarding either themselves or loved ones. For instance, I have told many people that their children have died, or that they have cancer, and it is hard. Most of the time, they take the news suprisingly well. Just think, wouldn't you want to know if you were in his shoes?
I think you need to tell him. Your brother in law is still a person, dementia or not, and he deserves to know the truth. He should be allowed to visit her to say goodbye, as it is his wife. I know that it will be hard (for him and you).
To tell him, I would do it when you are alone together, in a quiet place, without distractions. Allow him to process the information. Answer all his questions simply and clearly.
I wish you the best of luck!
