You must not have sexual relations with your father’s sister; she is your father’s close relative.
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 sister,.... His aunt by his father's side, an instance of which we have in Amram, Exo 6:20; and Maimonides says (i), an aunt was forbidden whether she was a father's sister in lawful wedlock or in fornication: she is thy father's near kinswoman; or, the rest of thy father (k); the residue of his flesh, one...
Lev 18:12-13 Marriage or conjugal intercourse with the sister of either father or mother (i.e., with either the paternal or maternal aunt) was prohibited, because she was the blood-relation of the father or mother. שׁאר = בּשׂר שׁאר (Lev 18:6, as in Lev 20:19; Lev 21:2; Num 27:11), hence שׁארה, blood-relationship (Lev 18:17).
Commenting on Leviticus 18:12-13