פירוש על ויקרא 18:18

‎ אל אחתה ‎[NEITHER SHALT THOU TAKE A WIFE] TO HER SISTER — both at the same time.

AND THOU SHALT NOT TAKE A WOMAN TO HER SISTER, TO BE A RIVAL TO HER, TO UNCOVER HER NAKEDNESS, BESIDE THE OTHER IN HER LIFETIME. Here Scripture explains the reason for the prohibition, stating that it is not proper for you to take a woman in addition to her sister, to be a rival one to another, for it is fitting that they love one another and not be rivals. He did not say so with respect to a woman and her daughter,²⁹⁶Verse 17. or a woman and her mother,²⁸⁷Further, 20:14. because they are “near of flesh” and therefore forbidden even after the death of the other [while in the case of two sisters, it is only during the lifetime of his wife, even if she is already divorced from him, that he may not marry her sister].

לא תקח לצרור, the Torah means that were it not for the competitive hostility which would result by the two sisters competing for the love of the same man, there would not have been a biological, genetic reason for forbidding such a union. Proof that this is the guiding motivation of the Torah is the fact that after the death of one sister, the surviving husband may marry the surviving sister.