(13) 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 versa 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.

18:1-30 Unlawful marriages and fleshly lusts. - Here is a law against all conformity to the corrupt usages of the heathen. Also laws against incest, against brutal lusts, and barbarous idolatries; and the enforcement of these laws from the ruin of the Canaanites. God here gives moral precepts. Close and constant adherence to God's ordinances is the most effectual preservative from gross sin. The grace of God only will secure us; that grace is to be expected only in the use of the means of grace. Nor does He ever leave any to their hearts' lusts, till they have left him and his services.Thou shalt not uncover the nakedness of thy mother's sister,.... Which is the same relation as before, an aunt by the mother's side; wherefore, if such a marriage was unlawful, this must also, and for the same reason:

for she is thy mother's near kinswoman; the same phraseology is used here as in the preceding verse; See Gill on Leviticus 18:12; and by the same rule a woman might not marry her uncle, whether by father or mother's side, the relation being the same, and this reaches to great-uncle and great-aunt; instances of women marrying their uncles, and men their aunts, among the Heathens, have been given, as among the Persians and Lacedaemonians by Herodotus (l), and among the Romans by Tacitus (m), but were, in his time, new things with the latter.

(l) Erato, sive, l. 6. c. 71. Polymnia, sive, l. 7. c. 224, 239. (m) Annal. l. 12. c. 5, 6, 7.