My ex and I have an 11 year old son together. We have a 60-40 custody split because of his job requirements. My ex got married 3 years ago and he has a 10 year old stepdaughter who lives with him full time. Our son and my ex's stepdaughter do not get along the best. For a while my ex and his wife decided to let things develop naturally but for around a year now they have grown concerned that the kids have not grown closer, appear to dislike each other more, and so they started pushing the kids together more.

This is not something I know a lot about. I don't interfere when my son is with his dad (unless it was something harmful to my son). Just like I expect him to do the same.

Not too long ago I took my son to see a movie he really wanted to see. It was him and three of his friends. We had a great time. My ex flipped when he found out. He told me he had warned the kids they were going to see it together. He told me I should have asked when I knew he was working on their relationship. He said I should be working with him here. I told him to hold up, that I was not about to ask permission to take my son to a movie.

He told me that's not working with him. That I should be making sure he doesn't want that saved for a bonding experience with both kids. He said our son manipulated me when I wasn't told he and his wife were planning to take both kids to that movie and had said something. I asked him what our son thought about that and he admitted our son hadn't wanted to go, and told him as much. He told me they need to do this stuff together or they'll never try.

I told him I do not need to ask his permission to do things with our son and I will not ask his permission. He accused me of not working with him. That I want their blended family to fail. I don't. But I don't want to deprive our son of fun stuff with me because he wants them to be sacred for his time only. And I don't want our son to be miserable doing all the fun stuff because he's forced to do it with the stepsister he doesn't like. And I think it's crazy to expect me to ask permission. Especially when I have our son more.

AITA?