Yevamot 30B

קנין גמור, קא משמע לן דלא כבית שמאי.

as a full-fledged acquisition. According to that opinion, the unrelated woman does not even require ḥalitza, since she is considered to be a rival wife of a forbidden relative. This Mishnah teaches us that the halakha is not in accordance with the opinion of Beit Shammai.

ולרב נחמן ⁠— קשיא דרב אשי! וכי תימא: הוא הדין דאפילו מת ואחר כך גירש ⁠— צרתה מותרת, אלא ״זו היא״ למעוטי מאי? למעוטי כנס ואחר כך גירש.

The Talmud raises a question from the opposite perspective: And according to Rav Naḥman, who deduced from the earlier Mishnah that the levirate bond is not substantial, the deduction of Rav Ashi that the levirate bond is substantial is difficult. And if you would try to resolve this in a similar manner and say: With regard to the ruling in the present Mishnah, which states that the sister’s rival wife, i.e., the unrelated woman, is permitted, the same is true even if Levi had died and then afterward the brother married to the other sister divorced his wife, there is a difficulty. If that is the case, then what does the phrase this is, cited at the end of the Mishnah, come to exclude? If we follow this explanation then the same ruling would hold true in all cases. The Talmud responds: It excludes the case where Shimon first married the unrelated woman and only afterward divorced the sister, as in such circumstances the unrelated woman is most certainly a rival wife of Reuven’s wife’s sister and is therefore not permitted.

הניחא אי סבר לה כרבי ירמיה, דאמר: תברא, מי ששנה זו לא שנה זו.

The Talmud states: This works out well if he holds in accordance with the opinion of R' Yirmeya, who said with regard to a seeming contradiction between this Mishnah and an earlier Mishnah (13a): The mishnayot are disjointed; he who taught this Mishnah did not teach that Mishnah. The earlier Mishnah established the principle that if a man was married to two women, one of whom was a forbidden relative, and he divorced the forbidden relative before he died, then the rival wife is no longer prohibited to the brothers.