A Noahite is permitted to marry his daughter. You may ask why Adam did not marry his daughter? He did it so that Cain could marry his sister, based on "The world will be built through kindness" in Psalms. A Noahite is also permitted to marry his sister. Didn't we just say that "The world will be built through kindness" allowed Cain to marry his sister, but otherwise, it would be prohibited?! True, but once it was permitted to Cain, it remained permitted.
Art: Mabel, Daughter of Charles Galloway by Edward John Gregory