Leviticus 18:12 Commentary - Ellicott's Commentary for English Readers (Old and New Testaments)

(12) Thy father’s sister.—According to the law which obtained in the time of Christ this prohibition not only extended to the father’s half-sisters, but even when they were begotten by the grandfather illegitimately. It is remarkable that Moses himself was the offspring of such an alliance, since his father Amram married his own aunt Jochebed, who was the sister of his father. (See Exo. 6:20.)