7 year old going out of town and is going to miss her brother's 2nd birthday?

My husband has a 7 year old daughter from his first marriage and we have 2 younger boys together. He just informed me (at 10:30) that his ex-wife told him today at 8pm that she is going out of town next weekend and wants to take daughter with her and her husband. Hubby just wanted to tell me before he forgot (which by the way, he forgets a lot when it comes to things about daughter and ex). The only problem with that is that next Sunday is our youngest son's 2nd birthday (hubby forgot about that being next weekend when he talked to ex). They don't have any legal custody agreement, but he is supposed to get daughter every weekend unless ex makes arrangements with my hubby for her to have daughter for a weekend. He told me that ex told him that she has had these plans for weeks, even though she is just getting around to telling him about it today. Now we have daughter this weekend, but last weekend daughter was at ex's because hubby and I both had to work and we thought it would be better for her to stay at her mom's than to spend Saturday with the babysitter. And the weekend before that daughter was at ex's because she had Saturday school because of all the bad weather and we thought it would be better for her to stay at her mom's than to only come to our house for 24 hours (school is 45 miles from our house, one way). Hubby did say that he will talk to ex tomorrow and see if they can work something out, but I don't see any good solution. We (me and daughter) have already made plans for going shopping on Friday after school and she is so excited about picking out a present for her brother all by herself. Hubby said maybe ex can come back early from her vacation so daughter can come to the birthday party on Sunday, yeah right. I need advice on what to think/do/say so that I can go back to sleep and not have this all on my mind all night. Do I have a right to be upset that ex didn't tell us when she made weekend plans with daughter and is just now getting around to telling us (I think she waits to tell him about her plans so that he can't say no to her, she has a habit of waiting until the last minute to tell him about plans she's made)? I know that I need to give hubby a chance to talk to ex about this tomorrow, but ex is going to be upset if we try to ruin her plans and he will just cave in if she gets upset. But ex made plans for a weekend before checking with us to make sure we didn't already have something planned, and this is our son's birthday.