Don't let it get to you, please. I promise you they are all like that! Your MIL meant well. I always worry about my kids when they're with someone else, coz they stress me out so much, but my MIL is the first one to reassure me that they all do it. They're all angels for others. You know what? If they weren't good for others, THATS when you question your parenting. You're the mother, its their job to push your boundaries. If they are behaving for others its because YOU'VE tought them the boundaries & what's acceptable & whats not. You've taught them to behave that way
They probably only fell asleep coz they were so bored