My pet theory on this is, porn is highly dependent on the “ooh la la” factor of hedonistic taboo boundary crossing as a crutch to cover for 95% of traditional studio porn being garbage with no sense of sensuality or eroticism. Make a big deal over the actress doing some act that “good women” don’t do to cover for the complete lack of chemistry.

Problem is, today’s taboo has a tendency to become tomorrow’s vanilla. Blowjobs used to be a thing only the dirtiest Parisian women of the night did, now they’re about as plain as plain gets. So porn ran into territory where all their previous taboos had lost that luster, and that left acts that were too far/gross for the average porn consumer’s taste.

Then comes incest, riding on a semi-highbrow wave of intrigue off GoT, with a workaround. The sex acts can be vanilla, but the presented relationship between the two characters is the hedonistic barrier. However, even though the whole thing is fictional regardless, they see going full incest as a bridge too far so the semi-incest step-relative sex becomes the “just spicy enough” middle ground.