You must not have sexual relations with your daughter-in-law. She is your son’s wife; you are not to have sexual relations with her.
These laws relate to the seventh commandment, and, no doubt, are obligatory on us under the gospel, for they are consonant to the very light and law of nature: one of the articles, that of a man's having his father's wife, the apostle speaks of as a sin not so much as named among the Gentiles, Co1 5:1.
Commenting on Leviticus 18:6-18
Thou shall not uncover the nakedness of thy daughter in law,.... 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...
Lev 18:15 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).