Actually, I am pretty sure that there ARE lots of people who would do that for a rehearsal dinner.
And then, they'd be remembered as the snippy couple with the screaming, grumpy kids, who had to get up and leave the rehearsal dinner (or worse, didn't) when the whole family's stress-induced behavior started to spiral downward!
I wouldn't get hung up on the injustice of non-parents offering advice. It's done all the time. Even the "Continuum Concept" was written by, you guessed it, a non-parent.
Personally, I would have laughed out out, and told my brother "Dude, after 5 hours in the car with screaming kids, you don't WANT me at the rehearsal dinner. I'll have hair, all grey, like the bride of Frankenstine, the kids will probably be pukey and gassy from all the screaming, and won't be able to speak because I'd have had to bite off my tongue to keep from being super bitchy to the first person to make some sort of comment that my stress addled brain perceived as snide. I'll miss being there (hey, I'm not opposed to a little white lying, personally), how about if I just owe you a dinner out with your new bride when y'all come to visit?"
People want what they want. It's obvious he probably really wants YOU there, so that's why he's fussing and saying those things. You know better, as to how much you'll really be able to enjoy and celebrate his marriage when you will have to endure 5 hours of sheer hell to get there. I'd let what he says go. He's just disappointed to not have you there, that's not a bad thing is it? You can still feel good about doing the right thing for your kids.
And just take him and his wife out to dinner next time they're out your way. Or start pestering them to babysit. ;>