You must not have sexual relations with the sister of your mother or your father, for it is exposing one’s own kin; both shall bear their iniquity.
Sins against the seventh commandment are here ordered to be severely punished. These are sins which, of all others, fools are most apt to make a mock at; but God would teach those the heinousness of the guilt by the extremity of the punishment that would not otherwise be taught it. I. Lying with another man's wife was made a capital crime.
Commenting on Leviticus 20:10-21
And thou shalt not uncover the nakedness of thy mother's sister, nor of thy father's sister,.... An aunt either by mother or father's side, against which the law is, Lev 18:13, for he uncovereth his near kin; as an aunt is to a man, and so an uncle to a woman, and both equally criminal; for it is a rule that holds good in all...
Lev 20:19-21 No civil punishment, on the other hand, to be inflicted by the magistrate or by the community generally, was ordered to follow marriage with an aunt, the sister of father or mother (Lev 20:19, cf. Lev 18:12-13), with an uncle’s wife (Lev 20:20, cf. Lev 18:4), or with a sister-in-law, a brother’s wife (Lev 20:21, cf. Lev 18:16).
Commenting on Leviticus 20:19-21