1. Go to the party. Make nice, smile, and go forward. Deflect all talk of future get togethers or of past time bombs. If you want some good lines--just start another thread with "What are your favorite "pass the bean dip" lines?". You will be amazed at the answers!!
2. Do not talk to SIL. No point to it. None.
3. Go to couples counseling. You and your DH need to work this out with a neutral third party.