Actually, Steam is perfectly fine with incest, in fact, there's plenty of games with it on the store
As long as the characters' look and age are over the age of 18/20/21 (I don't know, the age of consent on the State of Washington, where the Valve Corporation's registered office is based, is of 16 years old, but it is better not to risk by speculating) and there aren't way to desume the there are minors involved in sexual activities by the context (but's better if there aren't minor-like characters at all), like by the grade of the schools they attend (so the school should be at least a college or an university) the game pubblication on Steam, barring the unexpected, should be fine. Depiction of real people and illicit use of copyrighted material are forbidden as well, but those things are irrelevant in the case of this game
In reality, this eventuality has already been assumed in the past, but unfortunately nothing more was known about it afterwards (see
this post).