You are absolutely right, but you also have to avoid putting them on the defensive. Don’t be driven off the matter, but make it clear that your son’s approaching manhood make this beyond discussion. In spite of whatever good intentions they have, this needs to stop. Again: Not a counselor in the world will fail to back up your position. Express faith in their good faith, but draw the line where common sense requires it to be drawn. If your son is forced to give his reasons, he may have to admit feelings that feel profoundly and incredibly shameful to him. Don’t even go there. Stay on the island that says there are innocent reasons to want this, but that wisdom and elementary knowledge of the physical process of adolescence absolutely forbids it. I wouldn’t blame this on the divorce or your wife, though. Children go through adolescence without knowing what is going on with them, and they’re pulled apart by an inner process that they have no control over and a comforting past that they don’t want to leave, and sometimes they want things that are a bad idea. Don’t judge your son for having impulses with a power beyond his comprehension. If your wife drags her heels, remember that she has never been through a man’s adolescence, either. It is very different with girls, and she may be having trouble appreciating what your son has ahead of him. Could it be incest already? Yes. I think it would be an excellent idea to make sure your son has an opportunity to talk with a counselor about this topic, for that reason. If the situation is innocent, a counselor could also help assure him that whatever adolescent feelings he’s going through do not make him a monster of some kind. It would be a good idea to convince your wife that some sessions are in order so that your son will have a discreet person (read: not a parent) to help him sort out the feelings that his is going through. After all, the divorce means he doesn’t have you around to bring this stuff up with in the same way as if you and his mom were in a life of total agreement, and for your wife’s sake you’d like the son to have a neutral party to talk to. Again: Prudence would be on your side, even from the most secular perspective.