SAHM,
Yes, you should contact the in-laws, and expose his affair. Exposure helps to kill the affair.
He will be very angry when you expose. Be ready for him to say that exposing the affair is the end of your marriage. But the affair is the end of the marriage if you don't expose. You need to kill the affair, and exposing it is the best thing to do to kill the fantasy he has going on in his life.
Your in-laws can be helpful, but don't count on them to support you. It is often the case that they are weak-willed and support their kid (your husband), and don't hold up on the side of the moral thing to do.
Don't count on his being in love with the OW. He might think he is in love, but once exposure begins, his fantasy will start to cave in. That "perfect" romance? Not so perfect, because all of a sudden the reality of the ugly side of what he's doing will cave in on him. He has a price to pay, and one of the prices is the fact that he's been cheating and other people are now going to hear YOUR side of that.
Have you exposed to her family, husband???? Is OW married?
If she is married, let her husband know ASAP, because he can be an ally in the fight too.
If he thinks he is in love, there will be anger and resentment for exposure on his side, be expecting that, but do the best you can to stay in Plan A while he is angry over it. Ask him to go No Contact with the OW, and work on the marriage with you. Explain that the only way the marriage will work is if she is completely out of his life - no contact, no friendship, no nothing.
And if he won't go to counseling with you, go alone.
Call the Harleys today - they can help.
SB