That's your wife. According to the vows you took, your wife>your kids. IF I was in your shoes, I'd slap the fuck out of the kid who treated my wife "like shit", no matter what his age. And if he was 18+ and did like it- hit the fucking bricks kid.
You have 18 years to get it right, or at least set them on the right path. If he/she still fucked after then, it is what it is. Let them figure out their way.