Mr. or Mrs. Brady Bunch didn't hide the fact that they had previous children or relationships. Its very possible that the father lied to the mother about having the prior relationship. Its also possible that his previous behavior had nothing to do with the divorce, and it was his current reaction.

Either way, I think the author being a geneticist should pretty much understand that if you're trying to find relatives, you might actually find relatives.

I tend to think this should be opt-in, but as long as both parties have the box checked, it seems like perfectly valid functionality to me.