Read some Aquinas. Natural law is the moral order proper to human nature; the rights and wrongs that human beings, at least unfallen ones, would know instinctively.
Also, are you implying that marriage was around at the time of Adam? If so, could you give me a reference?
The book of Genesis refers directly to Eve as Adam's "wife". There's also a Hebrew apocryphal book (Book of Jubilees, I think?) which names all of Adam & Eve's sons and their sisters whom they took as wives.
Why would the “moral order” suddenly become dis-ordered? I am positively against moral relativism, as I trust you are. If inseminating one’s sister were natural at the time of Adam, why would it become un-natural to behave thus at some future time? Or are natural laws mutable?