I've always wondered if part of the reason for step-incest porn is that it's an easy and (porn-)logical way to explain why a hot person is in your house ready to have sex with you despite you putting zero effort towards making that happen. Same as how the pizza delivery guy thing was never about the pizza (not even in cases where the pizza is involved).

I'm sure some element of incest fetishism is at play too, but part has to just be that writing "help me step-bro I'm stuck" is easy boilerplate porn writing