AITA for refusing to call my niece anything else but her chosen name.

I (30f) recently got engaged and am happiest when I'm pregnant. I am currently not pregnant and have only been with my fiancé for under a year and have just started my period, but everyone knows that I will be trying again after the holiday. My family is split on the fact that I get pregnant so often with different men. My brother (34m) and I were talking about my upcoming wedding and we began laughing about past relationships, and he told me that he would like me and my niece (13f) to call him by his middle name. His middle name is John, so he wants us to call him John instead of his real first name Charles. <br><br>My niece and I both agreed that we had no problem with this, but I made it clear that we would only call him John and never Charles. My brother answered "Okay, I understand." But he clearly does not understand because a day later, he was yelling at me that I was trying to dictate what he can and cannot be called, and that it was in my power to force people to call him something they do not want to. I did not think this was true, so I asked a couple friends and family members if I was being an asshole for only allowing my niece and I to call him John. <br><br>Some people said no, you are not being an asshole, but most said you were. My brother has agreed to talk things out with me, but I'm really not trying to be an asshole, but I'd also rather just agree to call him John without any conditions than continue to be an asshole. <br><br>Tl;Dr my brother wants us to call him something other than his real name, and I only want to call him that name. AITA?