(i.e., thy step-mother), born to thy father,” is the half-sister by a second marriage; and the prohibition refers to the son by a first marriage, whereas Lev 18:9 treats of the son by a second marriage. The notion that the man's own mother is also included, and that the prohibition includes marriage with a full sister, is at variance with the usage of the expression “thy father's wife.”
verses 12-13
Marriage or conjugal intercourse with the sister of either father or mother (i.e., with either the paternal or maternal aunt) was prohibited, because she was the blood-relation of the father or mother. שׁאר = בּשׂר שׁאר (Lev 18:6, as in Lev 20:19; Lev 21:2; Num 27:11), hence שׁארה, blood-relationship (Lev 18:17).
Verse 14
So, again, with the wife of the father's brother, because the nakedness of the uncle was thereby uncovered. The threat held out in Lev 20:19 and Lev 20:20 against the alliances prohibited in Lev 18:12-14, is that the persons concerned should bear their iniquity or sin, i.e., should suffer punishment in consequence (see at Lev 5:1); and in the last case it is stated that they should die childless. From this it is obvious that sexual connection with the sister of either father or mother was not to be punished with death by the magistrate, but would be punished with disease by God Himself.
Verse 15
Sexual connection with a daughter-in-law, a son's wife, is called תּבל in Lev 20:12, and threatened with death to both the parties concerned. תּבל, from בּלל to mix, to confuse, signifies a sinful mixing up or confusing of the divine ordinances by unnatural unchastity, like the lying of a woman with a beast, which is the only other connection in which the word occurs (Lev 18:23).
Verse 16
Marriage with a brother's wife was a sin against the brother's nakedness, a sexual defilement, which God would punish with barrenness. This prohibition, however, only refers to cases in which the deceased brother had left children; for if he had died childless, the brother not only might, but was required to marry his sister-in-law (Deu 25:5).
Verse 17
Marriage with a woman and her daughter, whether both together or in succession, is described in Deu 27:20 as an accursed lying with the mother-in-law; whereas here it is the relation to the step-daughter which is primarily referred to, as we may see from the parallel prohibition, which is added, against taking the daughter of her son or daughter, i.e., the granddaughter-in-law. Both of these were crimes against blood-relationship which were to be punished with
