3. Mose 18:17

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 death in the case of both parties (Lev 20:14), because they were "wickedness," זמּה, lit., invention, design, here applied to the crime of licentiousness and whoredom (Lev 19:29; Jdg 20:6; Job 31:11).

Here is, I. A general law against all conformity to the corrupt usages of the heathen (Lev 18:1-5). II. Particular laws, 1. Against incest (Lev 18:6-18). 2. Against beastly lusts, and barbarous idolatries (Lev 18:19-23). III. The enforcement of these laws from the ruin of the Canaanites (Lev 18:24-30).

18:17 As with all examples listed in ch 18, marrying a woman together with her daughter or granddaughter was prohibited because of the disruption it would cause in the family order. It would violate the intimacy of the family group by forcing a wife to compete for her husband’s affections against a much younger woman. The penalty for this act was death by burning (20:14).