AITA for calling him a jerk for refusing to let his son be a flower boy at our wedding?

Me (24) and my fiancé (25) have been together for three years and are getting married in the summer. I have always wanted a romantic fairytale themed wedding with lots of flowers and a flower girl and boy. I have a niece that is 7 who has agreed to be the flower girl. When I was thinking about who would be great as a flower boy my fiancé's little brothers son, Julian (6) popped into my head. Julian is just a very sweet little boy and he is also very close with my niece who will be the flower girl. <br><br> Julian's dad "Tom" (34) is my fiancé's brother. Tom and his wife "Diane" (33) are going through a divorce. Tom and Diane have not been together for over a year and it seems like the divorce is almost finalized or so I have heard. Tom has been dating a woman named Emily (35) for about 6 months. Emily has two kids, girl (10) and a boy (12). Tom's ex wife Diane is also actually a florist. <br><br> So I asked Tom if Julian would be interested in being the flower boy and he said only if he isn't associated with picking flowers in any way since his mom is a florist. I thought that was a little weird but figured it would be fine. Great, so Julian will carry lanterns down the isle instead of flowers. So a few days ago I get a text from Tom saying that Emily thinks that it would be inappropriate for Julian to be a flower/lantern boy since her kids are not part of the wedding and they will be there. I asked Tom if he really expected his brothers wedding to be a family function for his girlfriend and her kids? He said that since they live together it's her family now too. I said fine Julian will not be the flower/lantern boy. <br><br> He texted back right away like I was being petty or something by taking back the offer and said that Julian would love to do it. So I said fine. But then I got a call from my fiancé a couple hours later saying that I really hurt Toms feelings. I said what? Tom asked Julian not to do it because his girlfriend didn't want her kids included but now he's mad at me? Tom called me later that evening as well to say that it was very mean of me to take back the offer then say fine. He said it was very mean of me to take the flower boy offer away. I told Tom that he was the one who initially said no because of his girlfriends kids so really it was Tom who took back the offer from his son not me. He said that he didn't want his sons uncle's wedding to be the first time Emily's kids meet his son and having his son involved would make it awkward. I told Tom that it would be no different with them all being guests and he said that it was different because his son would be involved in the ceremony. I thought it was ridiculous that he would give his girlfriend that priority and expect me to be understanding of Emily getting uncomfortable at his brothers wedding and ruining the wedding theme. He told me to stop being petty and said that he didn't understand why I was being mean to him and accused me of trying to start a fight with him since he didn't say his son couldn't be involved in the ceremony, he just said that it would make Emily uncomfortable. He then hung up on me when I asked him if he thought how uncomfortable it makes me that he's making such a big deal of his girlfriend and her kids at his brothers wedding.