The Noahide marital laws are derived from “...therefore shall a man leave his father and his mother and cling to his wife...,” - but not to the wife of his fellow, not to an animal, etc.
A Noahite is permitted to marry his daughter. If you ask, why did Adam not marry his daughter? - 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: Edward John Gregory- Mabel, Daughter of Charles Galloway