OP, to quote another thread somewhere here, "when you make plans without me, you make plans without me!" If your Mom insists on making plans using only your brother's dates, then she really can't be surprised when, inevitably, you can't always accommodate those dates. Too bad, so sad, Mom will get over it, or not. You should feel absolutely no guilt at refusing to ALWAYS give in and shuffle things around to suit them.
Also, my own mother carried on at holidays my entire childhood. I have very few memories of holidays where she didn't creat a scene, and I don't cherish those memories. I resent the hell out of the fact that she couldn't see (or care) how upset and angry and sad she made every body else feel - over and over again. IMO, it is extremely important to protect your son from that kind of behavior. You might be surprised (or not!) to find that he already would rather limit his contact with his grandmother, and would be perfectly happy to see his uncle and cousins separately, with less baggage and drama attached to the day.