I have a pet hypothesis that, on average, those into step porn are more likely not have grown up with a sibling of the gender they fancy. I reckon those of us who did are more likely to repelled by anything alluding to incest.
People who grew up with ACTUAL hot step siblings notwithstanding.
I was an only child and a latchkey kid, and am not just into sibling incest but full on incest. But yes, distilled, my fetish is for a functional, unneglected and much less solitary childhood. (Not turning into a pariah as soon as I started developing sexual interests in puberty might have also been nice.)
I suppose I also have the V. C. Andrews fetish which is taboo relationships that are formed as a cope due to extreme family dysfunctionality, since part of my fetish is the sex gets out of hand and when discovered, the drama explodes messily.
Curiously, my family on my mother’s side are hot, and my cousin was propositioned by Playboy in the 1980s as possible centerfold material. (She was also a high-school swim-team champion.) And yet, full Westermarck effect (reverse sexual imprinting) was active. I just couldn’t think about them in that way.