"Do not take your wife's sister as a rival wife and have sexual relations with her while your wife is living." (NIV)
You must not marry a woman in addition to her sister, to be a rival to her, having sexual relations with the second sister when the first one is alive. (AMP)
Neither shalt thou take a wife to her sister, to vex her, to uncover her nakedness, beside the other in her life time. (KJV)