AITA for not giving my son a birthday cake?

I (44F) am a single mom of three. I also have 2 step kids that live with me. I left their father 16 years ago after he cheated on me. I was awarded full custody. The kids’ father is married to my steps kids mother. My children are James (22) and twins Chris (17) and Kayla (17). He has two children with his new wife, Rebecca (17) and Nathaniel (11). <br><br>The kids are all friends. When we moved to a new state, my steps kids came with us, and they have been with me ever since, and are part of our family. Or so I thought. James, Chris and Kayla were always around to help when their steps siblings were young and needed to be fed, bathed, etc. <br><br>Chris had her birthday last week. We ordered a cake from the local bakery, but they called me to say that it had been cancelled by James. I called him to ask why did he do that, and he told me that he didn’t want to put his brother and sister out because they had to help pay for the cake, and they didn’t really have the money to spend on it. <br><br>I was very frustrated at his decision and told him that was not his decision to make and that we could afford a cake. He said that he didn’t want Chris to have a cake if his siblings couldn’t afford to chip in. I told him that they’re not his siblings and that they have never been a part of our family. <br><br>He said that I was being horrible and that he had every right to cancel the cake, since they’ve had to help with their basic needs since they were little. I was angry and disappointed. He and I haven’t been talking for days, and everyone at home is upset at the situation. My kids say that I’m being a terrible mom and that I shouldn’t have said what I said, since James has always helped take care of his stepsiblings. Aita?