What is this obsession in porn with step-sisters, step-mothers, and step-daughters? Honestly, it's kind of a turn-off. I know that the setup only lasts a few seconds out of the porn video's several minutes, but still, the premise spoils the effect.
Better relationship setups, in my opinion, also probably done to death, but at least not icky: neighbor, co-worker, fellow student, professor, student, nurse, doctor, babysitter, dog-walker, waitress, bar tender, room-mate, fellow musician, groupie, employer or employee (both bad ideas in real life), housekeeper, stripper, prostitute.
I say, less step relationships, more anything else.
I guess they just want to avoid sex between blood relatives.