A man may not marry his wife’s sister. If this were to happen, the women would rival each other. The term rival
(ṣrr) comes from the verb that means, to treat with hostility
(see, for example, Genesis 30:1-43). This prohibition is limited to the lifetime of the wife. If she does die, the man may marry her sister.
18 And you shall not take a woman as a rival wife to her sister, uncovering her nakedness while her sister is still alive.