I can't offer any info on specific legal stuff in certain jurisdictions; but, imo, there's no reason why it should be illegal. Provided everyone involved is a consenting adult, whose business is it?
What really gets me though is the fact that even "pretend" incest has pretty much been MIA from mainstream porn since the mid 80's. I guess due to that crackdown around that time on obscenity laws? Taboo is like one of the most successful pornographic film franchises of all time. Obviously a substantial segment of the public is interested in seeing this kind of the thing depicted. Bring it back I say!
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