Step dad's brother/sister or niece/nephew would really be stretching the definition of incest. Incest is between blood relatives and the reason it's so bad is because of inbreeding, sex with step-anyone isn't biologically speaking incest. If we wanna put step-parents and siblings under the rubric of incest for legal, social or cultural reasons fine, but don't bring in other more distant step-relatives. Assuming both are consenting adults It's perfectly ok for someone to date their step-dad's sister's son/daughter.
The thing is there is no leqal reason for stupid stepcest. I know of no place where the depiction of fictional incest in porn is illegal, even in places where the real thing is (which is by no means everywhere: in 22 countries, over 1/2 of the world by population, people can fuck their relatives and even breed with them if they want without getting arrested for it as long it's consentual). It's just stupid Mastercard being morality police with some stupid notion that it's fine to fuck step-relatives, but not biological ones, even though the whole point/allure of incest is risk (inbred baby risk, social pariah risk, legal risk, etc), which with stepcest doesn't exist (ie almost no one is going give that much of a fuck if your whole school finds out you were the one that knocked up your step-sister, if anything if she's hot and popular you'll probably be getting various forms of praise, envy and congratulations). If it's not Mastercard's risk I don't get why they give so much of shit to try to dictate what people can watch or pay for if there's no legal liability in doing so. What they do as far as censorship should be illegal, and the gov't should tell them to knock it off or be broken up into smaller companies via antitrust lawsuit like AT&T was (a duopoly is almost as bad as a monopoly). Until then what everyone should do (not just AVN devs and their supporters but porn producers in general) is move to Backed.by and start taking crypto payments, it's not that difficult, and then no one would have to censor themselves (just don't be Pornhub and avoid the real ch1ld p0rn).