Considering that the stepfather and stepdaughter are almost the same age, I don't see anything shameful or surprising about that. Sooner or later, when the wife should have left, the stepdaughter herself would have insisted on this action. Which is actually evident in the course of the video. The stepdaughter immediately exposed her breasts without thinking. Liked her intimate hairstyle - in times of fashion for naked pubes, such exhibits cause additional desire!
Something seems to me, if mom will practice such punishments for her son, his grades will become even worse. But in general, an interesting method of punishment, perhaps it would have been more effective if she hadn't let him cum.