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The papal magisterium (Pius XII) in Humani Generis taught that all human beings (except the original pair of course) are descended from the same parents. That is the Catholic teaching and it is in the CCC as well.
vatican.va/holy_father/pius_xii/encyclicals/documents/hf_p-xii_enc_12081950_humani-generis_en.html
37. When, however, there is question of another conjectural opinion, namely polygenism, the children of the Church by no means enjoy such liberty. For the faithful cannot embrace that opinion which maintains that either after Adam there existed on this earth true men who did not take their origin through natural generation from him as from the first parent of all, or that Adam represents a certain number of first parents. Now it is no no way apparent how such an opinion can be reconciled with that which the sources of revealed truth and the documents of the Teaching Authority of the Church propose with regard to original sin, which proceeds from a sin actually committed by an individual Adam and which, through generation, is passed on to all and is in everyone as his own.[12]
The Catholic Encyclopedia in its article on consanguinity takes the position that sibling relations are not contrary to the natural law.
*By the law of nature, it is universally conceded, marriage is prohibited between parent and child, for the reverential relation between them is recognized as incompatible with the equality of relations engendered by the bond of marriage. The universal sentiment of peoples is likewise opposed to marriage between all persons related in any degree in the direct line, thus between grandparent and grandchild.
HISTORY OF IMPEDIMENT
Because of the acknowledged derivation of the human race from the common progenitors, Adam and Eve, it is difficult to accept the opinion of some theologians that the marriage of brother and sister is against the law of nature; otherwise the propagation of the human race would have begun by violation of the natural law. It is readily understood that, considering the freedom of intercourse between such persons, some effort would soon be made (in the interest of the social welfare) to prevent early corruption within the close family circle by placing a bar to the hope of marriage. Hence among all peoples there has arisen a natural repugnance to the marriage of brother and sister. Some theologians suppose herein a positive Divine law, but it is not easy to point out any such early Divine enactment. Abraham married Sarah who was his sister by his father, though of a different mother (Genesis 11; cf. Genesis 20:12).*
newadvent.org/cathen/04264a.htm
For those unfamiliar with some of the terminology, “positive” above means it is enacted by the legislator (God in the case of a divine law) as opposed to being inherent in the nature of things (which is what the natural law consists of)
I agree with Pius XII that we are all descended from an original pair and I also agree with the Catholic Encyclopedia that sibling relations are not contrary to the natural law. When the Code of Canon Law was being assembled, this issue actually came up and what resulted was that the phrasing of the relevant canon was designed to simply say that the Church does not grant dispensations for siblings to marry without implying a judgment as to whether it was in accord with natural law.
vatican.va/holy_father/pius_xii/encyclicals/documents/hf_p-xii_enc_12081950_humani-generis_en.html
37. When, however, there is question of another conjectural opinion, namely polygenism, the children of the Church by no means enjoy such liberty. For the faithful cannot embrace that opinion which maintains that either after Adam there existed on this earth true men who did not take their origin through natural generation from him as from the first parent of all, or that Adam represents a certain number of first parents. Now it is no no way apparent how such an opinion can be reconciled with that which the sources of revealed truth and the documents of the Teaching Authority of the Church propose with regard to original sin, which proceeds from a sin actually committed by an individual Adam and which, through generation, is passed on to all and is in everyone as his own.[12]
The Catholic Encyclopedia in its article on consanguinity takes the position that sibling relations are not contrary to the natural law.
*By the law of nature, it is universally conceded, marriage is prohibited between parent and child, for the reverential relation between them is recognized as incompatible with the equality of relations engendered by the bond of marriage. The universal sentiment of peoples is likewise opposed to marriage between all persons related in any degree in the direct line, thus between grandparent and grandchild.
HISTORY OF IMPEDIMENT
Because of the acknowledged derivation of the human race from the common progenitors, Adam and Eve, it is difficult to accept the opinion of some theologians that the marriage of brother and sister is against the law of nature; otherwise the propagation of the human race would have begun by violation of the natural law. It is readily understood that, considering the freedom of intercourse between such persons, some effort would soon be made (in the interest of the social welfare) to prevent early corruption within the close family circle by placing a bar to the hope of marriage. Hence among all peoples there has arisen a natural repugnance to the marriage of brother and sister. Some theologians suppose herein a positive Divine law, but it is not easy to point out any such early Divine enactment. Abraham married Sarah who was his sister by his father, though of a different mother (Genesis 11; cf. Genesis 20:12).*
newadvent.org/cathen/04264a.htm
For those unfamiliar with some of the terminology, “positive” above means it is enacted by the legislator (God in the case of a divine law) as opposed to being inherent in the nature of things (which is what the natural law consists of)
I agree with Pius XII that we are all descended from an original pair and I also agree with the Catholic Encyclopedia that sibling relations are not contrary to the natural law. When the Code of Canon Law was being assembled, this issue actually came up and what resulted was that the phrasing of the relevant canon was designed to simply say that the Church does not grant dispensations for siblings to marry without implying a judgment as to whether it was in accord with natural law.