I don't want to advocate manipulation but asking questions concerning Joseph's dishonesty about polygamy to his first wife and his marriage of other men's wives and marriage of a 14 year old may prove more fruitful than just saying wow polygamy is messed up. Then here for a musical explanation of how those who yearn for a rational faith can resolve doubt through symbolical interpretation. I hate to say it, but I don't see this ending well. Don't let her try to bring in the missionaries to explain; remind her that she is an RM and knows all they do and probably much more. Moved into his house and it is hard to fit in.
Anything she learned about mormonsim, she wanted to learn on her own. I take offense to your statement about affairs. I explained to her that from my perspective, if a religious person does something good, you can't trust them because they're doing it for the wrong reasons: When an atheist does something good, you know they're not doing it for any reason other than to help someone else. Of course I have time for my SOs, of course I give of myself to them. Mormon theology is peculiar, yes, but the media frequently takes things out of context and misrepresents the religion.
I could go on, but this should give you the idea. You'll know pretty quick where she stands. Mind control is not child's play, and the Mormon church has real people's blood on their hands for the despicable damage they've caused. Are you going to keep the sabbath holy as a family, or is he going to take the kids out for pizza after church, leaving you home to observe alone. Sounds like classic "flirt to convert. Am I dating a douchebag.
You might start drinking to ease the pain. Some exceptions and valid reasons do exist for divorce but self righteousness is not on of them. The fact that you bring your query to Joanna Brooks rather than church authorities reveals much. Trust yourself, trust God and enjoy the beautiful relationship that you have. Do not make anything risque public. No one knows your situation the way you do. You can consider some positive activities and allow you to communicate and learn from each other. But that was also a possibility if he had married a non-mormon. One small thing to add here.