I am skeptical that this is all about some taboo. To start with, modern incest taboos are really weird. You peel away the layers and they go all kinds of very uncomfortable places for people. And, for some reason, the discussions almost always TOTALLY elide over the fact that the biggest real-world issue with incest is the fact that real-world incest is almost always based on fucked up familial power dynamics resulting in sexual assault/rape of the young and vulnerable.

The porn step-sibling stuff, I think, is mostly just bad writing. It is a convenient way to get otherwise-platonic adults in close quarters that generates sexual situations. It’s also a trope that lets you just skip any actual narrative-crafting. No different than all the absolutely ludicrous anime “locked in the gym storage room” tropes or that kind of shit. It’s just a hack writing way to skip to the part they want to film. It’s regular old hackery.

I think the fact that people seem to prefer a hacky story to no story at all for porn speaks to something I quite like about human nature. Even the porn we want elevated a little beyond itself.