You think incest is about inbreeding. It isn't just that. If you take that line to its logical extreme, then that means that it is okay if contraception is used or if same sex relations are involved.
I don't think it's healthy for fathers and adult sons to be having sex with each other, for example. Or that any of this becomes okay when a daughter turns eighteen (or whatever it is in your jurisdiction) and her father wears a condom. That is psychologically and socially unhealthy. Nothing to do with inbreeding.
If a school teacher repeatedly had sex with pupils a day after they left school (and were legal adults), that would raise numerous ethical issues. Consent isn't some kind of carte blanche or moral justification.
But no one is talking about actual incest, they are talking about a cheap storyline in a stupid online video made by consenting adults pretending (and actually) not related by blood.
If you have a moral objection to the content of the video, don't watch it, if you don't want to let people consume media that has objectionable themes, let's ban all violence as well, not just in porn, but movies, music, literature. No more Jason Statham, no more Daniel Craig, no more Dwayne 'the Rock' Johnson, no more Marky Mark, no more Eminem, no more Homer (sex and violence), no more The Bible (sex, incest, violence, rape, genocide, polygamy, slavery, child sacrifice).
"Step-cest" - a term I am sure has existed before, but I'm claiming it now, is make believe. And I'd bet 90+% of the watchers have it on mute.
The "step" thing is a get out clause. In the Netherlands, I've seen stuff advertised which doesn't even bother with that.