One of those daughters was Cain's wife. Yes, that would also be his sister ... You know Murphy's Law of Relativity, right? If you go back far enough, we're all related. Brother. Not only are you and I both descended from Adam and Eve ... we're both descended from Noah. How do you think Noah's grandchildren managed to "be fruitful and multiply"? They married their First Cousins. The Biblical prohibition on incest comes much later.
"If my parents didn't have any children, chances are I won't either."