just say in simple terms "NO I won't be doing that anytime soon, you're a brat and you're getting all angry because you don't get your way with me anymore and I don't care enough to waste my energy in these useless arguments. I'll be an adult, you try to do the same thing" ... and that's it.
Stand up to her bro, you're allowed, don't let anyone disrespect you, it's not just a lesson for you, it's a lesson for your kids as well, do you want your son to be treated by his wife that way when it's his turn at the relationship game? If you don't think the kids are watching, think again, they're recording all of it, at this early age, their brains are made to record everything.
Incidentally, this is great stuff.
"Always go straight forward, and if you meet the devil, cut him in two and go between the pieces." - William Sturgis, clipper ship captain, 1830's.
