Левит 18:16

Marriage with a brother's wife was a sin against the brother's nakedness, a sexual defilement, which God would punish with barrenness. This prohibition, however, only refers to cases in which the deceased brother had left children; for if he had died childless, the brother not only might, but was required to marry his sister-in-law (Deu 25:5).

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).

18:16 Sexual relations with your sister-in-law would be adulterous and disgrace your brother. The exception was the law of levirate marriage (see Deut 25:5-10), which called for a man to marry his brother’s widow in order to produce an heir who was considered the dead brother’s son.