AITA for complaining about being excluded at a family gathering with my nephew and niece still there?

I've been playing D&D for over two decades. I have two siblings, my younger brother who is 8y younger than me who I'll call Gary and my younger sister who is 10y younger than me. I've tried to get them to join a game with me over the years but neither were interested in gaming. Now all three of us are married and Gary has two kids, a 1y old and a 4y old. We live in a particularly bad area plague-wise and after about 6 months of not seeing each other were finally able to get together to celebrate the 1y old's birthday.Early in Gary lets it be known that he's running a D&D 5.0 game and has been playing online for 5+ months. Said game includes: himself, his wife, his wife's two sisters, our sister and her husband and one of Gary's friends. I was upset but didn't say so instead asking if I could join. Gary said no, his group was full. I wasn't happy but it wasn't like I could demand he kick a player out so I tried to not be super hurt. The problem was that, ringled by Gary, the group would not stop talking about their D&D game.No attempts to change the conversation would deter Gary from discussing this game that neither I nor my husband were part of and that I wasn't invited to. I kept quiet through the birthday dinner, the conversation after to let food "settle", the presents and the cake. Our parents were occupied with the grandkids and didn't care. My husband, who has high functioning autism, left the house to walk around outside due to not being able to join the conversation (to be fair, sometimes he has to do this anyways in large gatherings, but this time it was due to being excluded) and didn't say anything because he thought it was on him. It wasn't though- it's hard to join a conversation determined to be about a game you don't play in.After the cake, food, presents, etc. were done and both of the small kids were in the other room, I finally snapped and vented to my dad about how upset I was at Gary's behavior (this is NOT the first time I've been excluded by Gary. For example he likes to plan anime gatherings with my sister and BIL and his wife aloud when I'm there and not invited). Gary stuck with he had too many players but eluded to there being other reason.Speaking with my mom later she made it clear she thought I was wrong to mention this with my nephew and niece still there and that she doesn't think Gary's behavior was mean on purpose or bullying by exclusion as I feel. Neither child went to check out the noise or made any sign they heard me. I was not cussing. She made it clear that she thought I was out of line so AITA?Quick note: while I can and since have set up a D&D 5.0 of my own, it won't be with any of my siblings due to their schedules only accommodating one game. via /r/AmItheAsshole https://ift.tt/3li1DqY