Should I explain to My Broter in Law who has alzhiemers that his wife has cancer and is dying?

My sister is dying from cancer and is in the late stages. Her husband has alziemers, often repeats himself and often forgets where he has been. WE have been telling him that my sister fell and fractured her pelvis bone and will be in the nursing home for two more weeks. SHe did fracture her bone but we do not tell her husband that she is also dying from cancer. Do we let him know that his wife is dying or keep up the ruse and let him think that she will be home in two or three weeks or tell him the truth?