I don't see anything wrong in your conversation with your FIL. Your wife's health issues are likely due to the turmoil within her, not something that you are causing. She is ripping her family in two and the guilt must be killing her.
You have to be the strong one for your family, your wife is letting emotion dictate her path right now, and our emotions are never a good guide. Once the affair bubble bursts, your wife will begin to see more clearly, and logically.
