Leviticus 18:15

Thou shall not uncover the nakedness of thy daughter in law,
&c.] Shall not he with her in his son's lifetime, or marry her after his death: she [is] that son's wife;
and so one flesh with him, and who is of the same flesh and blood with his father, and therefore the nearness of the relation forbids such incestuous copulation or marriage: thou shall not uncover her nakedness;
or have carnal knowledge of her, whether in the life or after the death of his son, even then marriage with her is not lawful.