The commandment of yibum that we have been discussing throughout Massekhet Yevamot is carried out by an act of sexual relations between the brother (the yavam) and the widow (the yevama). According to the Torah, there is no need to first offer a ring or a marriage contract, as in a regular wedding, since this is effectively a continuation of the first brother’s marriage. Nevertheless, if one of the surviving brothers offers a betrothal ring to the widow – an act referred to in the Mishna as ma’amar – at least on a Rabbinic level, the yibum process is considered to have begun.
The Mishna on our daf offers a scenario in which there are three brothers, two of whom are married to sisters, while the third one is single. One of the married brothers dies, and the brother who is single offers ma’amar to the widow. Then the second married brother dies, as well.
In such a case, Beit Hillel rules that the surviving single brother cannot complete the mitzva of yibum with either widow. He will have to offer the widow of the first brother both a get (a divorce) and halitza; he will also have to perform halitza on the second brother’s widow. This is true because his ma’amar has created a situation of partial marriage between the yavam and first yevama. Since it is not a full marital relationship, we must view this as a situation where two sisters have become yevamot together, and he cannot perform yibum on either. Since there is a partial marital relationship, he must divorce the one who had received ma’amar.
The Mishna concludes by saying that it is such a case that leads people to say “oy lo (woe unto him), for he has lost his wife and his sister-in-law.”
Tosafot explain that this comment oy lo is specifically raised in this case because this man did nothing wrong and yet he discovers that all his efforts were for naught because of outside events over which he had no control. The Meiri suggests that this language stems from the poignancy of this story. The single man had an opportunity to marry – perhaps even two opportunities – and yet nothing came of it.