You have some ideas that are possibilities, but I think we need to think these over to make sure of the pluses and minuses of what could happen.
I am not real familiar with FB. Can you explain in a nutshell how it works? If you contact someone on FB, do their other friends or contacts see these conversations?
Let me know. My first instinct is to keep digging to get the OM's wife home phone or cell phone number or find out where she works or lives.
Keep digging here for now. Looks like some options could be opening up. As far as the FB way of contacting her sister or her, this sounds like a possibility. I would think it would be better to contact them is such a way that they would be curious enough to contact you to find out what you mean and are talking about. This would be better than just asking for a phone number. That probably wouldn't happen.