Charles Ellicott Commentary Leviticus 18:13

"Thou shalt not uncover the nakedness of thy mother`s sister: for she is thy mother`s near kinswoman." — Leviticus 18:13 (ASV)

Your mother’s sister. —Equally forbidden is the aunt on the mother’s side. The law that prevailed 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 period understood this last prohibition to apply strictly to marriages between nephews and aunts, but not vice versa to marriages between nieces and uncles.

They regarded intermarriage between an 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 neighbors, 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, 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 are common.