Ask them to confirm that they are supportive of the marriage. If yes, then ask your sister-in-law if she will implore your wife to end all contact with the OM, in a way that's acceptable to you her husband, and get into some good pro-marriage counseling with you.
Everyone's position needs to be the same: end the affair, and go work on your marriage.
Tell your SIL that you realize that you are not without faults in the marriage (but that you will NOT accept responsibility for your wife's foolish and selfish decision to have an affair!), and that -- once your wife agrees to 100% no-contact -- you are willing to address ANY AND ALL ISSUES with her and a MC.
Choc.
P.S. Be very careful when dealing her her family. It's good that they're supportive, but there's a reason why they say blood is thicker than water, kwim? Don't say anything negative about your wife that could get back to her. If you realize this, you can use this conduit for GOOD -- example, say -- if you mean it -- something like "You know, (SIL's name), I really love (wife's name), and I want to work on this marriage more than anything. I do NOT want a divorce, and (some genuine compliment about your wife -- like "she is still the most beautiful woman I've ever laid eyes on," or "she's such a strong person"). But until she ends all contact with this guy, we can't possibly work on our marriage, and I need everyone's help."
