L.R.
Bagging a dinner because of a concert: Not cool. But is it worth ditching this couple as friends? Probably not. If they keep on finding other stuff to do rather than come to your home? Yeah, that would become an issue. Can I assume here that they do not have children, or at least not young children, so they may not actually "get" that it was a big effort for you to do this?
Your being upset at expecting to dine with them "after the baby is born" and not doing so until the baby is five months old: You're overreacting. The friends might actually have felt that five months old is pretty tiny still. Did you interpret "after the baby is born" as being -- when? When the baby was three weeks old? Six weeks? Two months? Neither they nor you were clear about what "after the baby is born" means. I think your reaction may be more about hormones and emotions than about any real slight intended by these friends. Let this go.
Your mom and the beach trip: "Hey, mom, I have to say, this is the first I've heard of this and these plans for the beach were set back in November. We have rented the house for the entire week, too. If you really feel you must go see brother and GF, well, that's your choice, but I have to say that I am disappointed that such long-standing plans are getting changed within weeks of the trip, and disappointed that you will not be there with the grandkids the entire time as we had planned."
If that doesn't guilt her good, nothing will. But the point, more than the guilt, is TELLING her how you feel. She can be rude and hurtful about leaving, but you do not have to sit there and take it silently. I would not dredge up "you like brother better than me" stuff but I would ensure that she knew I was not happy with longstanding plans being changed.
Is it an option to invite brother and GF for that one weekend as well? If not, and if mom and dad do leave to see him, well -- as you note there are already issues between you and brother, so add this to the stack. But do not expect your mother to behave differently. You already know that she favors your brother, so why would you expect her to act differently in this one case? It's sad if she can't, but you and your family will still get a beach trip and can have family time without her and your dad -- a breather from them, if you see it that way.