About contacting the OM W and sister. So should the sister know about the affair at the same time as the OM W? At the same level of detail? Should I do that ASAP ? Or wait until later in the week?
If OM W spills the beans before time, she'll confront OM and might tip off W that I know already.
I would send the exact same message to OM W & her sister, and sooner, rather than later.
I also agree with robx (when we agree, there must be something to it ) about the family thing. Doing it in the way you have planned, could make you look kinda bad actually.
IMO, your best bet is if the OM is stringing your W along and has no intention of leaving his wife, and changes his tune when she find out. "Exposing" to your W isn't a big deal because she is planning to tell you anyway, and has practically been shouting it out on her FB status messages anyhow.