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.
Leviticus 18:15
