You shall not uncover the nakedness of your son’s daughter or of your daughter’s daughter, for their nakedness is your own nakedness.
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
The nakedness of thy son's daughter, or of thy daughter's daughter,.... A man might not marry his granddaughter, whether a descendant of his son or of his daughter, nor any further off descending from him in a right line, not his great-granddaughter, and so on; and if he might not marry his granddaughter, much less his own daughter, as Jarchi observes, for the relation is...
The nakedness of thy son's daughter, or of thy daughter's daughter, [even] their nakedness thou shalt not uncover: for theirs [is] thine own nakedness. (f) They are her children whose shame you have uncovered.