Leviticus 18:13 - Ellicott's Commentary On The Whole Bible

Thy mother’s sister. — Equally forbidden is the aunt by the mother’s side. The law which obtained in the time of Christ also defines this prohibition to extend to a mother’s sister or half-sister by the same father or mother, whether born in wedlock or out of it. It is remarkable that the administrators of the law during the second Temple understood this last prohibition strictly to apply to alliances between nephews and aunts, but not vice versâ to marriages between nieces and uncles. They regarded intermarriage between uncle and niece as an especially meritorious act, and interpreted the promises “then shalt thou call and the Lord shall answer” (Isaiah 58:9) to refer more particularly to the man “who loves his neighbours, befriends his relations, marries his brother’s daughter, and lends money to the poor in the hour of need.” This is in accordance with the fact that not only do we find that Nahor married Milcah the daughter of his brother Haran(Genesis 11:29), but that Othniel, the son of Kenaz, married his niece Achsah, being the daughter of Caleb, his father’s brother (Joshua 15:17; Judges 1:13). Hence among the Jews to this day intermarriages between uncles and nieces is of common occurrence.