What would you do?

I realize he is 88 and likely will never change. But I was in a situation once, more work related, that my boss was a real prick. Day in and day out. He also had some serious size to him so people feared him. One day I had enough, walked up to him, told him he was a real F****** A****** and I wasn't putting up with it anymore. I knew I was doing a good job, as are you by taking care of the MIL, and I didn't need his S***. I was surprised, he didn't even respond. But a funny thing happened that day. He changed his attitude towards me and my co-workers. May not be the case with your FIL, but some people just need to be told, really told.

In any case, good luck.