The logical explanation is rather more simple, I think (and rules out the barriers to incest).
God created a "very good" creation, and by that we would assume that the genetic structure of what He created was complete, fully operational, and without flaw. In that sense, there is no mandate against incest, for the main reason against it is propagation of genetic flaws.
AS the curse of sin multiplied, this genetic flaw became rampant because of the destructive patterns of humans, including their diet, the removal of certain features that were common to the early creation after the flood, and likely other concerns that we might only speculate about.
God THEN declared that incest was the propagation between blood relatives and set down a law to halt the process.
Before that, there was no law, as has already been pointed out, and indeed, all humans were descended first from Adam and Eve, both of the "same flesh" in more ways than the marriage relationship, and also through the sons and wives of Noah, who propagated the entire population of the earth as seen in the Table of Nations found in Genesis.
Can I back up the genetic issue scripturally? Only indirectly... In at least one instance we find Nephilim who were attempting to propagate with the daughters of men, seemingly in order to create a new race of people, i.e., introduce a new genetic structure through propagation. Also, there were the mandates to propagate within certain people groups and not with those who were outside the tribe, so to speak. I think that both are indirect genetic issues stipulated by God, knowing, of course, that the concept of genetics and passing down DNA was an alien topic until the past century or so, and more particularly this current generation, where some of the issues found in genetic structure have been revealed from the general revelation that is this observable world.
:applause::applause::applause::thumbsup: