Reminds me of a guy in one of my college classes who kept asking
cw: sv and general debatekid ickiness
"If the two parties consent, why should incest be illegal?" in an attempt to catch libs in some sort of ethical gotcha moment
He shut up real quick when I pointed out that because of the inherent power dynamics of a familial relationship, it's almost impossible to provide consent in cases of incest because that shit is coerced up the wazoo
Also that it was really fuckin weird that he kept bringing it up, because it was like the third time he did it in regards to three different discussions about consent
Straight white dudes are something else a lot of the time
Algorithm and marketing dynamics aside, incest is a wildly common problem in the US at least. In the social services work I've done it comes up frequently.
I thought about this a bit and, in addition to what you said:
Let's assume you have 2 siblings in their 30s who are romantically interested in one another, they're both independent financially and there's no power imbalance between them.
If you're in a position where both parties can consent, does the law even matter? Is the neighbor gonna call the police? Is the police gonna investigate and arrest them? I highly doubt it. I guess you can't get married.
Generally incest is a social construct, aside from power dynamics, even horizontal (e.g. siblings biological, step, whatever) incest suggests some dysfunction in the family dynamic when growing up, because we know things like the Westermarck effect make incest less likely.
But there's edge cases. Like people who were adopted and never met in childhood, like it's not really incest, though there's obviously genetic concerns which makes it advisable for them to not have kids (with each other).