Leviticus 18:9

Abroad; being born of your mother, while she was married to another. The marriages of brothers and sisters at the beginning, were authorized by necessity; but now they are the more to be condemned, as religion forbids them. (St. Augustine, City of God xv. 16.) Some Rabbins assert, that such connexions were lawful till the time of Moses. But St. Epiphanius (hær. 39,) maintains, they had been condemned long before. Seneca (St. Augustine, City of God vi. 10,) acknowledges that such marriages of the pagan gods were not right; ne pie quidem: and Plato says, they are hateful to God. The Romans punished them with death. Many barbarians do not, however, make any scruple to contract marriage with their children, or with their mothers. (St. Jerome, contra Jov. ii. 2.; Eurip. Hermione.) (Calmet)