As a step parent, I'd put aside your feelings and "be there" for your husband. I went to my SD's graduation and even went up to her Mom and shook her hand! My feeling was that I did not cause the divorce; therefore I have nothing to feel badly about.

Granted, I adore my S-kids, but that is beside the point. You are married to their Dad. Be there and do your best. We're here beside you!