She's very close to separated already (and that's actually a good thing) Don't force the issue in such a way that you will regret it later.
....here.....classic example. Have you ever known a woman who declared herself done with a boyfriend? What do you do. You don't trash talk the ex and tell her how bad he was because the second you do that and she gets back together with him then you're no longer welcome in their home. To connect the dots here, if Powersacker agrees with his wife that they "need to teach her a lesson" what is going to happen is the step daughter is going to say....."rents, who needs them?" and his wife is going to look at him and ask him why HE drove her baby away from them. That's NOT a place you want to be.
I mean....danger danger Will Robinson....read what you wrote...She is going to a wedding of her boyfriends relations in which he's not going to be there. and you think starting a conversation with her telling her she needs to straighten up and fly right in order to stay in your family is a good idea? You might want to re-think that, cowboy.