Yevamot 8B

למישרי צרה שלא במקום מצוה. מאי טעמא ⁠— אמר קרא ״עליה״: במקום ״עליה״ הוא דאסירא, שלא במקום עליה ⁠— שריא.

to permit a rival wife in a place where the mitzva of levirate marriage is not applicable. What is the reason, i.e., how is this inferred? The verse states: “With her.” In other words, it is in a place where the obligation “her brother-in-law will have sex with her” is applicable that a rival wife is prohibited, whereas in a place where the obligation “Her brother-in-law will have sex with her” is not applicable, a rival wife is permitted.

אמר ליה רמי בר חמא לרבא: אימא ערוה גופיה שלא במקום מצוה תישתרי! ולאו קל וחומר הוא? במקום מצוה אסירא ⁠— שלא במקום מצוה שריא?!

Rami bar Ḥama said to Rava: If the phrase “with her” is referring only to a case where levirate marriage is applicable, one can say that a forbidden relative herself should be permitted in a place where the mitzva of levirate marriage is not applicable. According to this interpretation, the prohibitions with regard to all those with whom sex are forbidden are in effect only when levirate marriage applies. Rava answered: And is it not an a fortiori inference? If these relatives are prohibited in a place where there is a mitzva, could they be permitted in a place where there is no mitzva?

אמר ליה: צרה תוכיח, דבמקום מצוה אסירא ושלא במקום מצוה שריא! אמר ליה: עליך אמר קרא ״בחייה״⁠ — כל שבחייה.

Rami bar Ḥama said to Rava: That is no proof, as the case of a rival wife herself can prove that this a fortiori inference is incorrect, as in a place where there is a mitzva she is prohibited and in a place where there is no mitzva she is permitted. Rava said to Rami bar Ḥami: With regard to your claim, the verse states: “And you should not take a woman to her sister…in her lifetime” (Leviticus 18:18). This indicates that all cases in which it is in the lifetime of her sister she is forbidden, even when no mitzva applies.