Baby mama drama

Ok, so my husband and I are expecting our first child together. We each have a child from our previous relationships, my son is 5 and his daughter is 1. My problem is his ex expects him to drop everything and be there when she needs a sitter or if her plans fall through she expects him to pick up the slack and pick up their daughter (even at 4am) I'm worried about when we have our baby, if he has to cancel our plans to be there for her or if I need him and then something comes up with his ex and he has to be there to get his daughter. I wish he would put his foot down with her and tell her we cant be there at every becking call because we both have jobs and lives that dont and shouldnt revolve around her and her schedule. But....I dont want to over step or upset my husband by saying that but its how I feel. What should I do?