Левит 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).

Here is, I. A general law against all conformity to the corrupt usages of the heathen (Lev 18:1-5). II. Particular laws, 1. Against incest (Lev 18:6-18). 2. Against beastly lusts, and barbarous idolatries (Lev 18:19-23). III. The enforcement of these laws from the ruin of the Canaanites (Lev 18:24-30).

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).