In the future, here's how you handle this situation. You've tried words and they don't work. You will need to use actions.
Next time your mother tells you "I talked to brother and we've decided..." you use a PP's suggestion of replying "I'll have to check my calendar to see if that works for me. I'll get back to you." Also, don't trust that what your mother says your brother might do is what he really wants to do. She said this time "He might contact you about doing something with you while the others are at the show." Yeah, he might. But he might not have said that to her. She might have just been giving you a situation that might happen. He never contacted you about it. Whether he does or not, you think about what you and DH want to do. What works best for you? Meeting on your driveway? Meeting in your house? Dropping DS off downtown and meeting up with your mom? You come up with the plan that is ideal for you. And then reply "What works best for us is..." and as long as you're not asking them to drive miles out of their way, they should agree to it, or maybe DS just doesn't go to that show (depending on how upset he'd be if you backed out).
Then, you communicate with mom *and with brother* so that they both get the same info. Because if you just tell your mom something, she's going to tell your brother something else. It's like the game of "Telephone." By the time he hears it, he might get frustrated with you because he thinks, for example, that she told you that they're all arriving at 6pm. Your mother doesn't need to be the information gatekeeper. She feels important being the hub of all knowledge. Shut that down and communicate directly with your brother.
Finally, if you're expecting them at 6:30, you don't open the door if you hear a knock before 6:29pm. You have other things going on and you weren't sure you'd be home. Maybe you're in the shower. Maybe you're napping. Maybe you're still at the grocery store. You were expecting them at 6:30. Maybe a knock at 6:05 is a salesman and you don't like solicitors. No need to open the door or even check the peephole. If they are on your doorstep for a half hour, they can wait in their cars with the heater on. Natural consequences for not communicating with you.