Ketubot 82B

מהו דתימא: מצוה דרמא רחמנא עליה עבדה, והשתא תיקום עליה באיסור אשת אח, קא משמע לן.

The Talmud explains: It is necessary lest you say that he has performed the mitzva God placed upon him by means of levirate marriage, and now that he has divorced her she should once again stand in relation to him with the prohibition proscribing a brother’s wife, which was her status before the mitzva of levirate marriage came into effect. The tanna therefore teaches us that since he performed levirate marriage with her, the prohibition proscribing a brother’s wife no longer applies at all.

ואימא הכי נמי! אמר קרא ״ולקחה לו לאשה״. כיון שלקחה ⁠— נעשית כאשתו.

The Talmud asks: And say that indeed, the prohibition proscribing a brother’s wife should be in force once again. The Talmud explains: The verse states: “And he will take her to him to be his wife” (Deuteronomy 25:5), which indicates that once he has taken her, she has become like his regular wife in all respects.

בלבד שתהא כתובתה על נכסי בעלה הראשון. מאי טעמא ⁠— אשה הקנו לו מן השמים.

§ The Mishnah taught: She has the status of his wife in all respects after levirate marriage, except that the responsibility for her marriage contract is upon the property of her first husband. The Talmud inquires: What is the reason for this? It is that from Heaven they acquired a wife for him. Since he did not choose her but married her by force of a Torah commandment, he is not obligated to set aside for her a marriage contract of his own. Rather, he relies upon his brother’s marriage contract.

ואי לית לה מראשון, אית לה משני. כדי שלא תהא קלה בעיניו להוציאה.

The Talmud adds: And if she does not have anything from the first husband, e.g., if he owned no property, she nevertheless has a marriage contract from the second one, for the same reason that any wife is entitled to a marriage contract in the first place: So that she will not be demeaned in his eyes such that he will easily divorce her.