You must not have sexual relations with both a woman and her daughter. You are not to marry her son’s daughter or her daughter’s daughter and have sexual relations with her. They are close relatives; it is depraved.
These laws relate to the seventh commandment, and, no doubt, are obligatory on us under the gospel, for they are consonant to the very light and law of nature: one of the articles, that of a man's having his father's wife, the apostle speaks of as a sin not so much as named among the Gentiles, Co1 5:1.
Commenting on Leviticus 18:6-18
Thou shall not uncover the nakedness of a woman and her daughter,.... That is, if a man marries a woman, and she has a daughter, which is the man's daughter-in-law, after the death of his wife he may not marry this daughter; for this daughter is of the same flesh with her mother, who became one flesh with the man she married, and therefore his...
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.