C. F. Keil and Franz Delitzsch on Leviticus 20:19-21

Lev 20:19-21

No civil punishment, on the other hand, to be inflicted by the magistrate or by the community generally, was ordered to follow marriage with an aunt, the sister of father or mother (Lev 20:19, cf. Lev 18:12-13), with an uncle’s wife (Lev 20:20, cf. Lev 18:4), or with a sister-in-law, a brother’s wife (Lev 20:21, cf. Lev 18:16). In all these cases the threat is simply held out, “they shall bear their iniquity,” and (according to Lev 20:20, Lev 20:21) “die childless;” that is to say, God would reserve the punishment to Himself (see at Lev 18:14). In the list of punishments no reference is made to intercourse with a mother (Lev 18:7) or a granddaughter (Lev 18:10), as it was taken for granted that the punishment of death would be inflicted in such cases as these; just as marriage with a daughter or a full sister is passed over in the prohibitions in ch. 18.