What should I do? My husband's ex doesn't want me at my stepson's graduation.?

My husband and I have been together for over 12yrs. My stepson is graduating high school next week and his mother doesn't want me to come. She's NEVER liked me. They live in another state and my husband wants us to go on vacation there but stay in the hotel room while HE goes to the graduation. I've never done anything to her OR my stepson but her attitude doesn't surprise me since she's hated me from day 1. My husband and I are currently working on our relationship and I don't trust him, entirely. What would you do in a similar situation? My stepson wants me to come but his mom doesn't and my husband seems TOO content w/HER decision. He just wants me to brush it off and let them go. Should I? He hasn't even tried to get a reason for her attitude or tried to talk her into letting me come. This makes me more suspicious. How should I handle this?