35 It is waste of time to observe Faustus' remark, that Abraham falsely called Sara his sister; as if Faustus had discovered the family of Sara, though it is not mentioned in Scripture. In a matter which Abraham knew, and we do not, it is surely better to believe the patriarch when he says what he knows, than to believe Manichæus when he finds fault with what he knows nothing about. Since, then, Abraham lived at that period in human history, when, though marriage had become unlawful between children of the same parents, or of the same father or mother, no law or authority interfered with the custom of marriage between the children of brothers, or any less degree of consanguinity, why should he not have had as wife his sister, that is, a woman descended from his father?
For he himself told the king, when he restored Sara, that she was his sister by his father, and not by his mother. And on this occasion he could not have been led to tell a falsehood from fear, for the king knew that she was his wife, and was restoring her with honor, because he had been warned by God. We learn from Scripture that, among the ancients, it was customary to call cousins brothers and sisters. Thus Tobias says in his prayer to God, before having intercourse with his wife, "And now, O Lord, You know that not in wantonness I take to wife my sister;" though she was not sprung immediately from the same father or the same mother, but only belonged to the same family.
And Lot is called the brother of Abraham, though Abraham was his uncle. And, by the same use of the word, those called in the Gospel the Lord's brothers are certainly not children of the Virgin Mary, but all the blood relations of the Lord.
Source: Reply to Faustus the Manichaean (New Advent)