We have already learned that when a man dies with no children, the Torah commands one of his surviving brothers to perform yibum (levirate marriage). We also learned that there is an option for someone who chooses not to perform this mitzva, as the Torah allows him to do halitza (see 25:7), a ceremony that releases the woman from her relationship with this family and allows her to marry anyone that she wants.
What if one of the surviving brothers performs the halitza ceremony and then chooses to marry his late brother’s wife? Does the original prohibition of marrying one’s sister-in-law remain, or was it removed when the opportunity to marry her through yibum was permissible?
Reish Lakish argues that the punishment of karet – which is the standard punishment for someone who performs an act of incest or adultery – remains intact for all of the brothers, with the exception of the one who performed halitza. For that brother, only a simple lav remains, for which he would be liable to receive lashes, but not karet, since the Torah ( 25:9) puts him in the category of an ish asher lo yivneh et beit ahiv – a man who refuses to rebuild his late brother’s family – and once he has refused, he is forbidden (on the level of a lav) to do so. For the other brothers the original prohibition – which was much more severe – simply remains.
Rabbi Yohanan rules that the severe punishment of karet no longer exists, neither for the brother who performed halitza, nor any of the others. He argues that the brother who performed halitza did so as the shaliah – acting as an agent – for the other surviving brothers. Therefore they are all placed in the same situation.
The Rashba explains the disagreement between Reish Lakish and Rabbi Yohanan as being based on the question of how the mitzva of yibum is to be perceived. According to Reish Lakish, the original prohibition of marrying a sister-in-law remains intact, even in a situation of yibum. Fulfillment of the mitzva pushes aside the prohibition. Rav Yohanan, on the other hand, views a yibum situation as one that removes the prohibition entirely. Even though halitza will still be needed to sever the relationship between the widow and the surviving brothers, there is no longer the potential punishment of karet.