Go look at fan forums for House of the Dragon, discussing the most recent episode. There was an incest scene and the gist of fan reaction was "hnnnng that's hot".

I don't mean publicly. I mean in conversations where interest in taboo fetish is common and encouraged.

And I have no doubt that there are people who get excited about step-sibling fetish. I just figure that if it was as common to want step-sibling porn as it is to find it, then I would have met (or heard from) someone by now who does.