Having talked to her myself I can see she's one those teens that knows everything there is to know and nothing is ever her fault, because really, the entire world revolves around her. I really don't think she's mature enough to be married and I'm still surprised her dad gave the guy his permission.
That aside, I can't believe she's running her mouth behind your back. Actually wait, I can. If she keeps acting the way she is she wont have anyone left standing in her corner when the shit hits the fan later on, and she'll deserve the cold shoulder she gets. What a little twat -.-
But it's okay, let her be a stupid brat. While she's making an ass of herself you're being the civilized and mature adult she's supposed to be. She's not worth the bullshit.