Dear Eric: My son recently announced that he and his wife are divorcing. Shortly after that, I found out my son had an affair with the daughter of my ex-husband’s wife. Technically, the other woman is his step-sister but they only all met as adults, so it’s not cringe-inducing incestuous. Apparently, my ex-husband knew about the affair while it was occurring, and never said anything to our son, in order to protect their relationship.