C. F. Keil and Franz Delitzsch on Leviticus 18:15

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).