In the current situation, you're damned if you do, damned if you don't as far as whether to attend the reunion.
At this point I would focus on the more important relationship: you and your future hubby. That needs to be strong before you can ever address the MIL issue. I would base your decision on whether to go to the reunion on how your hubby really feels about it and how that will affect you as a couple. It sounds like the reunion may be more important to your future hubby than any "party" aspect of the reunion.
THEN, and more importantly, I would focus on some couples counseling to find ways to discuss things between the two of you and how you can stand together as a couple in the future. You need to each feel that you can express your honest feelings, come to an agreement together, and trust that the other will honor that agreement. A counselor can help give you the tools to do this.