I need answers from Step moms.

How do you handle your SOs previous family? I really like my SOs daughter but he's supposed to have her 4 days a week and his ex is supposed to have her 3 days a week. Problem is his ex will ask him to take his daughter on her days too alot. So he winds up having her 6 out of 7 days most of the time. So heres my problem. . His ex will tell him she wants to have their daughter and she will pick her up etc but then 9/10 times she will call him with some excuse as to why she can't have their daughter that day. And by last minute I mean he will make plans with me and my son and then have to cancel them to go and get their daughter from wherever his ex has left there daughter be it at daycare or her mums house or wherever. I understand his daughter comes first but I'm starting to think his ex is just doing this to screw up our plans. It's starting to realllly bother me and last night I blew up at my SO about how I rarely see him and how he just goes along with whatever his ex wants. He wants to move in with me and is talking about us having a child together. How do I overcome my problem with how his ex is behaving? It's making me want to end our relationship. I have no problem with him having his daughter I always make sure she feels included (she's 3) but it's more like his ex dictates what we can do even though she rarely even sees her own daughter.