Leviticus 18:18

Neither shalt thou take a wife to her sister, to vex [her], to uncover her nakedness, beside the other in her life [time].

Ver. 18. A wife to her sister,] i.e., Any two women together: compare Ezekiel 1:9. Here polygamy is flatly forbidden. In which sin many of the patriarchs lived and died; not through any impiety, the Lord testifying that their hearts were upright, but merely through the mistaking of this text, as it may seem, taking the word "sister," for one so by blood, which was spoken of a sister by nation, as those clauses, "to vex her," and "during her life," do evince.