that's where I'm coming down on this. Two kids, one 6 and the one a little older. And the older one claims the 6 year got in and flew off. It certainly wouldn't be the first time in history when a kid lied about something.
It might not be a lie exactly. When child no.2 says "The balloon's gone up, and my brother was in it!", he might have meant "I saw my brother in the balloon. He was messing with it. Then he got out and started messing with the tether, and then it got loose, and my brother ran off."
I'm suggesting the child was assigning blame "he did it", as children do, rather than alerting parents to a serious incident in progress.