פירוש על יבמות 3:1

ארבעה אחין. חולצות ולא מיבמות – since both of them are levirate relations one to the other. The first [brother] who undergoes levirate marriage came in contact with the sister of his levirate relation and is like his wife.

Introduction Mishnayoth one through four are all one long mishnah dealing with a case of four brothers, whom we will call, Reuven, Shimon, Levi and Judah, two of whom married two sisters, Rachel and Leah.

ב"ש אומרי יקיימו ובה"א יוציאו – In the Gemara (Talmud Yevamot 28a) reverses them that the School of Shammai states that they should be put away (i.e., divorced), and the School of Hillel states that they may remain wedded, for in every place, the School of Shammai is stringent and the School of Hillel is lenient except from those known places where the School of Shammai is more lenient and the School of Hillel is more stringent (see, for example, the first five Mishnayot in Tractate Eduyot, Chapter 5, of which this text is taken), and thus we say, that generally, the School of SHammai in the place of the Schol of Hillel is not a Mishnah, meaning to say, that wherever we find in the Mishnah that the School of Shammai are lenient and the School of Hillel, where it is their manner to be lenient, but and it is found now that the School of Shammai stands in the place of the School of Hillel, it is not a Mishnah and it is a corrupted text and it is necessary to reverse it unless it is from the known Mishnayot which are from the leniencies of the School of Shammai and the stringencies of the School of Hillel. And in Tractate Eduyot (Chapter 5, Mishnah 5), in the name of Rabbi Eliezer, it enumerates the leniencies of the School of Shammai and the stringencies of the School of Hillel.

Four brothers: two of whom were married to two sisters, if those who were married to the sisters died, behold these must perform halitzah but may not have yibbum. If they preemptively married them, they must divorce them. Rabbi Eliezer said: Beth Shammai holds that they may retain them, and Beth Hillel holds that they must divorce them. Reuven and Shimon marry Rachel and Leah and then both die. Rachel and Leah are technically liable for yibbum or halitzah with either Levi or Judah. The mishnah teaches that neither Levi nor Judah may have yibbum with either sister, because each sister is the sister of his z’kukah (a woman with whom he is liable to have either yibbum or halitzah). In other words Leah is liable to have yibbum or halitzah with each brother as is Rachel. They are both considered to be “z’kukah” to both brothers. If Levi or Judah were to have yibbum with either one, he would be having relations with the sister of his z’kukah, which is forbidden just as it is forbidden to have relations with one’s wife’s sister. Even after one sister has had halitzah with one of the other brothers, thereby ending her status as z’kukah, the other may not have yibbum because any one woman is forbidden to him at the time when she becomes liable for yibbum, remains forbidden forever.

If the brothers did marry the sisters, against the ruling in clause one, they must divorce them. Although the marriage is valid and therefore requires a divorce, it is forbidden and therefore a divorce is mandatory.

According to Rabbi Eliezer, the previous ruling is only according to Beth Hillel. Beth Shammai disagrees and rules that the marriage may be upheld.