From the Epistle of St. Gregory the Great, to Augustine, Bishop of the English (Epistles, Book XI, Epistle LXIV), A.D. 601

Question 5. Whether two full brothers ought to accept individual sisters, who are begotten from a stock remote from them.

Response. This may be done in every way. For nothing is found in the sacred utterances which seems to contradict this chapter.

Question 6. To what generation ought the faithful to be coupled in marriage with their own relatives, and whether it is lawful to be coupled in marriage with stepmothers and kinswomen.

Response. A certain earthly law in the Roman republic permitted that the son and daughter, whether of a brother or of a sister, or of two full brothers or two sisters (paternal cousins, first cousins, cousins' daughters) be mingled. But we have learned by experience that offspring cannot grow up from such a marriage. And the sacred law (of Moses) forbids revealing the shame of your kinship. Hence it is necessary that now the third (the children of cousins) or fourth generation of the faithful ought licitly to be joined to one another. Alexander II, in his epistle to the clerics of Naples on computing the degrees of consanguinity, reads: whence it is necessary that now four or five generations be coupled to each other. For the second, which we have mentioned, must abstain entirely from each other. But to be mingled with a stepmother is a grave crime, because it is written also in the law: You shall not reveal the shame of your father. That also signifies that another thing too is implicitly called forth, namely, to the words of the Apostle, 1 Cor. ... With a kinswoman (that is, the wife of a brother) also it is forbidden to be mingled; because through the prior union she has been made the flesh of the brother.