Charles Ellicott Commentary Leviticus 18:14

"Thou shalt not uncover the nakedness of thy father`s brother, thou shalt not approach to his wife: she is thine aunt." — Leviticus 18:14 (ASV)

Thy father’s brother, thou shalt not approach to his wife.—That is, according to the ancient legal interpretation, a nephew is to have no sexual relations with her during her husband’s lifetime, nor marry her when his uncle is dead. Those who transgressed this law had not only to bear their sin, but were doomed to die without children. (See Leviticus 20:20.)