Leviticus 18:18

18 “ ‘Do not take your wife’s sister as a rival wife and have sexual relations with her while your wife is living.

Read Leviticus 18:18 Using Other Translations

Neither shalt thou take a wife to her sister, to vex her, to uncover her nakedness, beside the other in her life time.
And you shall not take a woman as a rival wife to her sister, uncovering her nakedness while her sister is still alive.
“While your wife is living, do not marry her sister and have sexual relations with her, for they would be rivals.