Imagine you and your partner do make sexy videos but then your respective parents hook up.

The videos were made when you were not step siblings, but now you are.

This is essentially my understanding of the origin of the trope. When I was growing up, both me and every single kid I knew’s parents were already or in the process of either getting a divorce or remarrying.

So situations like Clueless and Drake & Josh were the norm, and it makes the trope all the more hilarious.