Through error, error, error, and trial, I have learned it is best to keep your in laws at arms length. If they want to work things out with their daughter, they will. It is best to not try and play peacemaker. These people were looney way before you came into the picture and you are in a truly unfortunate position. However, you are not a therapist, nor Buddha, nor Ghandi. Speaking from experience, if you want to protect your own sanity and emotions, just leave them be. It can be very difficult at times.