Leviticus 18:7

Father, with whom the daughters must not have any connexion, as Myrrha had with Cymoras. (Metam. x.) (Haydock) --- All relations in a right line are excluded for ever, according to the emperor Justinian. The reason of these various impediments is, 1. That God's people may not resemble infidels, who permitted such things, ver. 3. The Persians married their own mothers, daughters and sisters. (Clement of Alexandria, strom. 3.) Semiramis married her son Justin. Cleopatra was both mother and wife of the two Ptolemies, Philometor and Euergetes, or Physcon. (Tirinus) --- The Egyptians took their sisters to wife for a long time, by the authority of their laws, and in imitation of Isis. (Diodorus 1.; Clement, recogn. 9.) Solon permitted people to marry their step-sisters by the same father, and Lycurgus only those by the same mother. (Philo ad 6. præc.) 2. By this law, the bands of society are strengthened, and families become connected. (St. Augustine, City of God xv. 16.) 3. Disorders which would easily take place under the same roof, on the prospect of a future marriage, are prevented. 4. The contrary practice would often prove contrary to order and decency, as the son would be raised above his mother. These regulations seem to have been made from the beginning, or at least from the time of the deluge; since the nations not subject to the law of Moses, are condemned for the transgression of them, ver. 24. See Genesis xix. 33. (Calmet)