I've always favored a simpler explanation for the rise of incest roleplay porn: you can take literally any porn scenario, change a few of the words, and suddenly it's "incest porn" in addition to whatever other genres it already belonged to. Not only is it free taboo that doesn't require your actors to physically do anything they might not be comfortable with, it also means any viewers not into that can easily tune it out since it's otherwise a standard porn scene.