I have no idea why it became so popular, but to try to make a real distinction from stereotypical plot-based porn, maybe it’s the difference between sex with a stranger vs sex with someone you know? Are people listening to porn with the volume up? If it’s muted, it might just seem like an awkward will-they-won’t-they(but they definitely will) of a legitimate friendship turned sexual.

It might feel a bit more passionate to watch someone have sex with someone the actor supposedly already has a relationship than a random pizza boy or mechanic etc. I don’t know how popular “watch me have sex with my girlfriend” videos are, but I feel like people who are substituting relationships with porn would specifically avoid those because of their shame and jealousy of not having a partner? But if you’re watching someone so without shame that they’d have sex with a step sibling, the viewer doesn’t have to think about their own needs and issues as much. They get to be more “pure” when comparing themselves to incestuous siblings while still getting off.

Theres also probably something people find hot about taboo shit in general too. Maybe it’s as simple as “Stranger is to Secret-sex-in-public-porn as Known-person is to Step-sibling-porn”.

But I’m ace, so what the fuck do I know.