Leviticus 18:15

“Thou shall not uncover the nakedness of thy daughter in law, etc.] 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.”

“18:15 Just as having sexual relations with your father’s wife would dishonor your father, having sexual relations with your daughter-in-law would dishonor your son. The law called for daughters-in-law to be treated like natural daughters (Exod 21:9; cp. Gen 38:1-30).”

“Lev 20:12; Gen 38:18, Gen 38:19, Gen 38:26; Eze 22:11 Leviticus 18:16”

“Sexual connection with a daughter-in-law, a son's wife, is called תּבל in Lev 20:12, and threatened with death to both the parties concerned. תּבל, from בּלל to mix, to confuse, signifies a sinful mixing up or confusing of the divine ordinances by unnatural unchastity, like the lying of a woman with a beast, which is the only other connection in which the word occurs ( Lev 18:23). Leviticus 18:16”