My boyfriend of 2.5 years has two older brothers, both of whom are in relationships (that began after ours). Yesterday, him and his two brothers and their girlfriends (who live out of town) took their mother out for a special dinner.
I was not invited, as he assumed I “was busy” without even asking. I live pretty much on the way to the restaurant, and had literally nothing going on. Needless to say, I was pretty upset and still am…. yet he refuses to apologize as he feels he did nothing wrong. I only found out about said dinner when I saw photos of everyone that their mom posted on Facebook, when I texted him. His reply was “I guess you can still come”. By the time I got into my car and called, they were leaving. Am I right to be upset? I feel like shit because in my opinion I look like the jerk for not being there.
LemonVibe 
Consider this a big red flag. It’s not enough to break up over, but you should keep your eyes open since he doesn’t seem to care all that much about you or your feelings.
I would be pissed off too. It’s one thing to say “I know you’re busy, so it’s ok if you can’t come”, but to not tell you entirely is pretty shitty. I agree with ^, while not breakup worthy, he may not cherish your relationship that much or hold you in high importance in his life.