I'm not sure what good it would do to expose to OW's family. No way will they take your side in this since you mean nothing to them. If OW has an H then I'd say definitely expose it to him. But her family would be of any help to you.
Now, exposing to your H's family is another matter. Does your H have a good relationship with them? Does he look to them for support and advice? If so, exposing the A to them might help bring the A to a quicker end. Or it might not. There really is no way of knowing.
The real question is, what do YOU need to do to feel better about yourself? For me, I exposed to my W's family because I wasn't going to lie to cover up for her and because she was filled with so much self-hatred and shame that she needed help.
There are no easy answers here, and every sitch is different.
Me: 33 WAW/MLC: 33 M: 4+, T: 10+ Separated: Nov 08 A#1: Oct 08 - Jan 09 (exposed and ended) A#2: Feb 09 - ? 1: http://tinyurl.com/mrmistakes 2: http://tinyurl.com/ckch9t 3: http://tinyurl.com/stillwaters3