My Brother In Law Has Started Accusing My Sister Of

My Brother in law has started accusing My Sister of having a affair 50 years ago and has become obsessed with it. She has told over and over it never happened but He insist it did. He has made live a living hell for Her. He was diagnoised with Prostate cancer several months ago and has had surgery for it. He is 77 years old. I am wondering what has caused this delusion