You must not dishonor your father’s brother by approaching his wife to have sexual relations with her; she is your aunt.
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 shalt not uncover the nakedness of thy father's brother,.... Which Gersom understands of committing sodomy with him, on which account he was doubly guilty, partly because of lying with a male, and partly because of uncovering the nakedness of his father's brother; but it rather seems at first sight as if the sense was, that a woman should not marry her father's brother, that...
Thou shalt not uncover the nakedness of thy father's brother, thou shalt not approach to his wife: she [is] thine aunt. (g) Which your uncle discovers.