The more you attend family events that include the affair partners, the more your WW will believe you are accepting the fact her affair will continue. How much time has already lapsed since discovering their affair? How many times have you been around BIL? And now, the family is going together on a trip.

Here is what I think you will do. You will do nothing. You will hurt and keep hoping your W will eventually end an affair that has been going on for years, and you will hope against hope she won't leave you. In the meantime, you will try to look the other way.

It is your life, Tate, and it is your choice to live in a lie. If your W had any sense of decency and/or respect for your parents (we already know she has none for you), she would excuse herself from joining the family vacation. However, she sees it as an opportunity to be around the BIL, and her selfishness will over-ride her sense of doing the right thing.

It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!