You won't find one, because that is by far the most reasonable explanation, usually assumed by almost all serious students of Scripture. (well, the conservative ones anyway).
Right, he was pretty serious in Deut. about Sabbath-keeping, and a million other commands not given to Cain....why assume it was his "sister" anyway? It could have been his great-great-great niece for all we know. The age difference would have been insignificant. Have you even put any serious thought into this???
He forbids it certainly at this point, that doesn't mean he always did.
His essential nature doesn't change, but his policies and requirements for his creatures might.
That sort of distinction escapes you.
Why assume that?
Right, because they are aware of the fact that there is no evidence of it being forbidden until explicitly verboten in the Mosaic Torah.
You are inventing laws and doctrines WITH ZERO SCRIPTURAL SUPPORT.....just as you accuse KJVO'S of doing. That is pointed out to you occasionally, but you only care to be consistent about a particular dying and increasingly irrelevant issue.
We are, you were speaking of despising teaching of doctrines with ZERO SCRIPTURAL SUPPORT.....such as teaching commands which are not in Scripture such as forbidding incest amongst pre-Torah peoples, and inventing creation histories out of whole cloth....both being doctrines with zero Scriptural support.
Along with the KJVO myth, you should reject such man-made doctrines.
While the man-made origin of KJVO is well-known & of recent origin, we just DON'T KNOW much about man's earliest generations. But God said in Malachi 3:6, "I am the Lord; I don't change." So, I believe God at His word.
Of course, SOME things have changed, in ways. For instance the requirement for MAN to sacrifice animals for forgiveness of sins was completely fulfilled by JESUS, so WE don't sacrifice animals, but depend upon Jesus for sin forgiveness.
But we simply don't know about the earliest generations, so we really can't rule anything completely out.