I have to ask, "Why are you not talking directly to your brother regarding events that include your brother?". Why is everyone going through mom and then when you all receive different stories, you are surprised?
For the theater event, you and brother should have talked the day before and discussed the plans. "Ok, bro, what is happening as far as you know on your end per mom? Oh, this is what I was told. Ok, now how do we want this to go?"
This way there are no surprises because mom did not tell you anything.
As far as brother and invites, why not just come out and ask him? "Bro, please don't take this wrong way, but I am seriously confused by your invitations. You say you don't expect us there, but is it tha tyou are just inviting kids and we can come if we want to, or do you not want us there or would you like us there, but you are leaving it up to us per our schedule(s)? I'm sorry, but it is very confusing and I would just like to clarify you real intent so that there are no hurt feelings on either end if we are missing something".
Exactly what I was wondering. You and your brother are adults, OP, and both of you know how your mom does things. I say it's time to take away that power and deal with these situations yourselves. Honestly, I don't see why your brother couldn't email/text/call you about details such as when he would get to your house.