IANAL, but I'm pretty sure you're grossly misinterpreting those ToS. Incest may be illegal in some states, but depictions of incest are not (or all those companies producing incest porn would be in hot water, wouldn't they? It's fine because no actual incest is occurring). Underage sex is prohibited just about everywhere, but US legal precedent is that depictions of underage sex are only banned if they look too realistic.
All of those companies producing incest porn use "step" relations, if you listen to the dialogue. I know cuz I watch most of it.
EDIT: The ones in the US do, at least.
SECOND EDIT: Not sure what legal precedent you're referring to, but this was the last case I know of where the simulated depiction of minors and incest was involved, and the dude got three years in prison for having comics:
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In said article, it also quotes the US Code covering it:
"According to United States Code, Section
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, it is illegal for:
Any person who … knowingly produces, distributes, receives, or possesses with intent to distribute, a visual depiction of any kind, including a drawing, cartoon, sculpture, or painting, that -
(1)(A) depicts a minor engaging in sexually explicit conduct; and
(B) is obscene; or
(2)(A) depicts an image that is, or appears to be, of a minor engaging in graphic bestiality, sadistic or masochistic abuse, or sexual intercourse, including genital-genital, oral-genital, anal-genital, or oral-anal, whether between persons of the same or opposite sex; and
(B) lacks serious literary, artistic, political, or scientific value"