You must not have sexual relations with your mother’s sister, for she is your mother’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 mother's sister,.... Which is the same relation as before, an aunt by the mother's side; wherefore, if such a marriage was unlawful, this must also, and for the same reason: for she is thy mother's near kinswoman; the same phraseology is used here as in the preceding verse; See Gill on Lev 18:12; and by the same...
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