Just read your thread and jumping in on the in-laws question.
I know others recommend not involving the WAS's family but I also felt like I had to simply explain my position. I was supposed to go with BF to his parents' house for Christmas and already had a slightly tenuous relationship with his mother. I was afraid that not showing up and him not offering any explanation (which he didn't) she would assume that I wasn't going because I don't like her/them/the family which is completely untrue.
So I ended up sending a short email along the lines of, "I want to explain my absence for the holiday. BF is having an affair with a coworker. I still love him very much and am willing to work on our problems but I cannot do this unless and until he ends his affair."
BF's family is extremely non-confrontational and non-communicative so I didn't expect a response but his mother did email back about a month later. She said I had become part of the family over the years and hoped things would work out for the best for both of us.
That's my experience for what it's worth.
If you love somebody, set them free. http://tinyurl.com/2empx2g
