When being a stepmom really sucks...

This all started a couple years ago, DCFS was involved in a child abuse case where mom had been accused of severe child abuse and neglect, while my husband was out of town. Anywho court had ended, mom was returned custody and mom decided that she just didn't want to let him see his 2 year old son anymore. So after saving up for a year for a lawyer and all that fun stuff we finally got joint custody established (real mom and my husband were never married and apparently DCFS doesn't situate custody). This all came into place in July... I was 25 weeks pregnant. I guess the frustrating part was that it was very noticeable that I was pregnant and I even let the lawyers know that I could go into labor during visitation.
Anyway we asked mom if we could trade weekends as I'm set to have this baby on the 23rd. Her answer is that she has plane tickets to leave town that are non refundable and we are irresponsible for not taking Aiden on scheduled visitation. I don't even know what to say! This is the same woman who is still on felony probation for Child Abuse, I was unaware you could leave state lines for vacation... so there's a chance my husband will have to miss the birth of his daughter because we don't have anyone who can baby sit, everyone decided to leave town this weekend. What should I do?