Does she have a right to be upset?

The other night my boyfriend's daughter and son (6 and 4) called him while we were driving. He chatted with them for a little while until his son asked about my sons (5 and 3) who were also in the car. His son asked to talk to mine so we handed him the phone and the boys were having a blast talking about video games and such. While they talked, I told my boyfriend I wish we had his children more because I know he misses them and I'd love nothing more for our children to be close since we're all a family now since recently had our own son. Their mom however refuses to let them come over. My boyfriend hasn't seen his kids in close to 3 months which is upsetting for everyone. My boyfriend wants to see his son and daughter but his ex always comes up with some excuse to prevent it. For example, the kids were supposed to come over for our baby shower to hang out with family and friends. My ex even let me have our boys for a few hours (the party was planned on the day he had them). But his ex refused saying a baby shower is no place for children because of alcohol. Mind you my boys were there as well as both my nieces and literally no one drank a drop and even if they had I was pregnant and 100% sober and more than capable of watching the kids. Then she admits that she had already planned something for the day of the shower even though she had known the date months in advance. Anyway, back to the other night. His ex texted him upset that his son talked to mine because it took away from my boyfriend talking to his children. While I understand it a little bit, at the same time our children are all friends and I feel like shouldn't they get just a little time too? I could totally be in the wrong here but I just wish we all had more physical time together as a family. My boys miss his kids and I know that feeling is mutual. My boyfriend and his ex are currently in the middle of custody and she is supposed to be letting him see the kids at least 4 times a month but she's refuses to let him see them saying she can't trust him. Which I just don't understand. I live with the man and he doesn't do anything that could put them in any bad situation. He's great with our son and my boys. I can't help but feel she's being petty and using the kids as pawns. The situation just sucks.