I think if OM is divorced, it's a tougher call. Ordinarily, I would say that the OM/OW's spouse has a right to know, as they would otherwise be the ONE person in the equation (of the four) that DIDN'T know what was going on, and I think they have the right to know the truth and make their own decisions accordingly.
Re: her family. Do you think they would be supportive of the marriage?
What was your previous (pre-her-affair) marital style -- overly controlling, or overly accommodating/pleasing?
