Tanach · Rishonim on Tanach · Ramban · Torah
Ramban on Leviticus 18:18/רמב"ן על ויקרא י״ח:י״ח
אלִצְרֹר לְגַלּוֹת עֶרְוָתָהּ עָלֶיהָ בְּחַיֶּיהָ וּבְכָאן יְפָרֵשׁ הַכָּתוּב טַעַם הָאִסּוּר, יֹאמַר שֶׁאֵינוֹ רָאוּי שֶׁתִּקַּח אִשָּׁה אֶל אֲחוֹתָהּ לִצְרֹר אוֹתָן זוֹ לָזוֹ, כִּי הֵן רְאוּיוֹת שֶׁתִּהְיֶינָה אוֹהֲבוֹת זוֹ אֶת זוֹ, לֹא שֶׁתִּהְיֶינָה צָרוֹת. וְלֹא אָמַר כֵּן בְּאִשָּׁה וּבִתָּהּ (ויקרא י"ח:י"ז) וְאִשָּׁה וְאִמָּהּ (ויקרא כ':י"ד), כִּי הֵנָּה שְׁאֵר וַאֲסוּרוֹת אֲפִילּוּ לְאַחַר מִיתָה:
1AND 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,296 or a woman and her mother,287 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].