Maybe this would be a valuable tack to take. Tell me, then: please lay out your case for what you believe “the unity of the human race” means, and why it’s salient in this discussion, and why brother-sister sexual activity – which is condemned everywhere and by everyone – doesn’t harm your notion of “the unity of the human race.”
I’ve led out the case multiple times. As far as what ‘unity of the human race means,’ it is in the Bible, the CCC, and Tradition, and I have referenced this in prior posts multiple times as well I believe. Accordingly, please go back and reference those posts and if your still not quite sure what the ‘unity of the human race’ means than let me know. Hint: it begins with the creation of Adam and Eve in Genesis 2.
According to the reply of the Pontifical Biblical Commission, June 30, 1909 which is official Church teaching, it is not possible to call into question the following Church teachings contained in Genesis 1-3 that touch upon the foundations of the Christian religion among which is the unity of the human race:
Question 3: Is it possible, in particular, to call in question the literal and historical meaning where there is question of facts narrated in the same chapters [Genesis 1-3] which touch the foundations of the Christian religion: as are, among others, the creation of all things by God in the beginning of time; the special creation of man; the formation of the first woman from the first man; the unity of the human race; the original felicity of our first parents in the state of justice, integrity, and immortality; the command given by God to man to test his obedience; the transgression of the divine command at the instigation of the devil under the form of a serpent; the degradation of our first parents from that primeval state of innocence; and the promise of a future Redeemer?
Answer: In the negative.
As for the question under discussion here, see ST, Supplement, Q. 54, art. 3 which Aquinas sums up in the following paragraph:
‘Accordingly it is clear from what has been said that consanguinity is by natural law an impediment to marriage in regard to certain persons [parent/child], by Divine law in respect of some, and by human law in respect of others.’
From Gorgias: Look… I get it that there are various sources of law. Your quote only affirms my point: brother-sister marriage is precisely one of the impediments that Aquinas calls “by Divine law”!
Did you read the quote from Aquinas? You have asked this at least once already and I explained it. Do you see he mentions three kinds of law in the quote? The Divine Law he is talking about is revealed Divine Law such as the Old Law (Mosaic) from which he quotes in the body of the article or the New Law of Christ.