Yevamot 29 - Statement of intent
Imagine two brothers married to two sisters, who have a third brother, a bachelor. One of the married brothers dies, and the bachelor now has to do a mitzvah of yibum or chalitzah with the widow.
Although the mitzvah of yibum simply means that the brother lives with the widow, the Sages established an additional "statement of intent to marry," which looks like a betrothal. The bachelor acts as proper and makes this statement. However, then the second brother dies. What is the situation of yibum now?
Beit Shammai sees no problem: he can keep his wife, to which he made a statement, and the other one - she is his wife's sister - does not need either a yibum or a chalitzah. Beit Shammai regards a statement as a valid betrothal, making them husband and wife. Usually, before they can live together, there must be a marriage ceremony, but because yibum already allows them, it is not required.
However, Beit Hillel says that the statement only looks like a betrothal, but being that it is not a real one, but only an action instituted by the Sages for propriety, she is not his legal wife. Therefore, he cannot keep her - because there is another sister with whom he is connected with the bond of yibum, nor can he marry that sister because of the same bond, plus his additional statement; in both cases, he is as if marrying his wife's sister, which is forbidden. Therefore, he must do a chalitzah with the first sister and give her a divorce document (Get) because of his prior statement. He then needs to do chalitzah with the other sister. He is thus left without any wife through no fault of his own.
Art: The Betrothal by Jose Juarez
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