How to persuade father-in-law not to buy another house?

Father-in-law is caring for my mother-in-law who has mid-stage/late-stage Alzheimers (doc predicts she has another 3 years based on his experience). FIL wants to sell their house because it is two-story and buy another house. He is 80 and she is 76. We kids are trying to persuade him to put that equity into savings and rent instead because they are going to need the cash soon. He is adamant that that would be wasteful. He currently has a second mortgage anyway with a payment that would be about the same as rent so it's not a cash flow issue. How do we show him that sinking all that equity into another house with another mortgage is a really bad idea? He says they are both going to live to be 100. Yes, denial is not just a river in Egypt.