Leviticus 18:18

Lastly, it was forbidden to take a wife to her sister (עליה upon her, as in Gen 28:9; Gen 31:50) in her life-time, that is to say, to marry two sisters at the same time, לצרר "to pack together, to uncover this nakedness," i.e., to pack both together into one marriage bond, and so place the sisters in carnal union through their common husband, and disturb the sisterly relation, as the marriage with two sisters that was forced upon Jacob had evidently done. No punishment is fixed for the marriage with two sisters; and, of course, after the death of the first wife a man was at liberty to marry her sister.

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).

18:18 The word rivals (Hebrew tsarah) vividly describes the relationship between Leah and Rachel (Gen 29:16–30:24) and Hannah and Peninnah (1 Sam 1:6); in both cases, the fellow wives were driven to bitter rivalry for their husband’s affections.