Pseudo-incest (step parents, usually) is a huge and super popular category in written erotica, actually. Erotica is almost always aimed at women, but it's interesting that they share that in common with porn video sites (which tend towards male users), particularly since there are a lot of other very popular erotica genres that aren't nearly as popular on porn sites (human cow, shape shifters, billionaires, etc.) and vice versa.