I've played about 60 VNs on Steam, and saying that incest is allowed on Steam would be a guess on my part because what I remember is opening a form to edit the relationships in the games I have. Aren't they all like that?
Steam has no issue with incest.
Steam's biggest bugbear is schools (high school in the states specifically) as there would be a very, very small window of time where a student would still be at 'school' and
also be 18. So it's a blanket ban on Steam, regardless of what the dev says the girls ages are in a disclaimer. This is no difference to the current witch hunt by authorities/customs on sex dolls also (despite there being no actual laws in place).
What you say the age is, what the manufacturer says and what it looks like...are 20 different things as it cannot be proven and is subjective to the person looking (unlike a real person who may look underage but
can prove they are not a minor). However, in a 'school' setting, most likely they would not all be 18 years old (as start 'college' at 18 at the end of summer...which would be a few months after already having left 'high school').
Why this will never be on Steam without some (more) major rewrites.