Odd, if appeasement never works, why do I see so many parents trying it?
That would be the stupid, bad parents I mentioned.

It's just that most were busy, one was mopping the floor, two were doing dishes and packing up food, etc. etc.
So only you have to deal with out-of-control kids? Only you have to care that some of them may get hurt playing with scissors and X-acto knives? If the boxes weren't yours, you wouldn't have tried to keep the kids out of them?

When parents let their children run wild at family functions, counting on relatives to keep them out of trouble, those parents are letting and expecting you to act as a parent in their place. Which is supposed to mean they back you up when you do. Anyone (your sister) who wants to excuse kids' behavior as "just being kids" should also expect the parents to keep a closer eye on them.

You don't dump your kids off on people who didn't ask for the responsibility and then criticise them for taking that responsibility seriously.