Hmm, agree that Patreon's indication to Zorlun (the dev) that any reference to family link, even "step-mother" or "step-mother", would have been considered incest and brought to a ban of his account, is a completely off the scale interpretation. Effectively, even by USA legal standards, if there is the "step", which means no blood link, there cannot be incest.
A known Hollywood director ended up having a relationship with the younger women he and his wife had adopted - no blood link, no incest.
About the cousins, though, I am not sure at what level you are talking about, but I know at least one country, not USA, that consider first degree cousins with a blood link as still being incest (notice the "first degree by blood", though, second degree, is already OK).
For the rest, yep, if you allow a comment from far away, for the country that AFAIK still has the biggest porn industry by turnover in the World, and in which teenagers (and I mean below 18) with kids are not the most diffused thing, but far from a rarity, is funny trying to pretend that people below 18 cannot be physically developed enough to have sexual urges and sexual activity.
Whether they are mature enough mentally and emotionally, is a different matter, but for that, there are even people in their 40s for whom the first description coming to mind is not "mature"
.
Puberty can be a risky period from that point of view, but you don't avoid a risk by pretending it does not exist.
About the "statutory rape", AFAIK is a bit more messy.
Most countries seem to have eliminated, or never had, "statutory rape" (one above 18, another below, is automatically rape, in practice was meant to protect girls from "predators' and AFAIK enforced basically only when the above 18 was a man), while in parallel, the most diffused age of consent in the World is 14-17yo (but there are others).
I gave a look, and it seem in USA is really a mess
Basically, if two adolescents are romatically involved, before they move beyond kissing, they'd better get a specialised lawyer consultation
The age of consent varies in theory between 16 and 18, but when you cross with "statutory rape" (a term apparently in USA they do not like, so they use different ways to call it), it is even more messy.
The situation varies from the "if the older one is not in a position of power over the younger one, it's ok if there is age of consent"; to cases where it seems if one manages to get a lawful marriage (? with legal guardians/parents consent, maybe ?) in reality they may accept even an age for one of the two below the age of consent (?); or a state where a 14 year old could have sex with a 18-19 year old being still considered in the limits of the law, although the age of consent in that state is 16, but only if the older one has not solicited the sexual activity; to a State where any sexual intercours with someone below 18 is illegal, unless they are married, even if both are under 18 and are in full agreement, though in that case classed as misdemeanor; in another state, apparently age of consent is 16, but then a 13yo can consent to a 15yo, and a 12yo can consent to a 13yo (?), and age of consent becomes 18 if the older person is in some positions (tutor, coach, authority above the younger, etc.).