I think if you don't want to go to the concert badly enough to suffer the inconveniences that will result from doing so, then what you are really telling yourself is that, well, you just don't really want to go to the concert all that much.
That's a perfectly reasonable stance.
I can tell you that I probably wouldn't want to go to a concert the first night I arrived in Hometown. And especially if I'd been traveling w/a toddler in tow. I'd want to settle in. (Unless the concert were one I really, really wanted to see.)
I will say that kids are really NOT harmed by one night of not sleeping well, or feelings stressed, or whatever. (And your kid might surprise you by being far more flexible than you think, once he's required to so do.) And that your mom setting things up one day on the first night does NOT mean things have to stay that way.
And hey, if your mom does what she wants, the way she wants, and it turns out to be a problem, then you've actually gained ammo to say, "See, I really do know better than you."
It's ONE night. It's just not a permanent thing. And your son might suffer for an evening, but my own opinion is that nobody promised ME any stress-free life, and my kids didn't get that guarantee either.
In fact, I'm of the belief that facing the really-not-that-dangerous stress of sleeping in a strange place, not getting to sleep, etc., etc., is actually one of the important ways kids grow and learn and adapt. From "suffering" comes growth. Muscles get stressed, and then they get stronger; bones get stressed from activity, and then they get denser; brains get stressed by learning and experiences, and then they get smarter/faster/more agile. Ditto character.
So if you do really wish you could go to the concert, my advice would be, go. You son's "suffering" won't be that severe, and he can be inconvenienced in order to make it possible for you to do something you enjoy.
And I'd say, then tell your sister, "I don't want to go badly enough." Then she can't argue with that.