The only people who are going to be hurt if you don't attend are the MIL and FIL and you. Your wife won't give a fig about your absence...but you have to decide what you want to do. Maybe you can visit w/the MIL and FIL on a different day?? If that is better for you.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
