C. F. Keil and Franz Delitzsch on Leviticus 18:17

Lev 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).