I'm thinking if I'm dropping the kids off at MILs tonight, would this be a good time to expose W's affair to them? Or would SIL already have let the cat out of the bag?
RULE #1 OF MARKETING:
Always position YOURSELF, before your competitor does it for you.
If you want to control the message, A, you will do this yourself. Just be honest with them, tell them how much you respect them and would never want to lie to them, and that you love their daughter very much and want to fight for the marriage, but you're finding it very difficult when you're being disrespected this way.
Talk as if EVERY WORD will get back to your wife. Just speak the truth, in love, and you'll do fine.