The Torah in Parashat Ahareh-Mot (Vayikra 18:18) commands that one may not marry his wife’s sister “Be’hayeha” – “during her lifetime.” This means that once a man marries a woman, he may not marry or have relations with that woman’s sister, even after he divorces his wife, until after his wife dies. Once his wife passes away, he is permitted to marry her sister.
This prohibition forbids both betrothing the wife’s sister, and having a relationship with her without marriage. One who betroths the sister or has an intimate relationship with her outside the framework of marriage has transgressed this prohibition.
The Sefer Ha’hinuch explains the reason for this prohibition as rooted in the Torah’s concern to maintain peace and harmony among people. Certainly, the Sefer Ha’hinuch writes, the Torah seeks to avoid friction among family members, who are naturally drawn to love and cherish one another. If two sisters marry the same man, this would likely lead to jealousy, friction and competition, thus ruining the relationship between the two sisters, and for this reason, the Torah forbade marrying two sisters. In the interest of maintaining peaceful relations among sisters, the Torah commanded that one may not marry his wife’s sister unless his wife has passed away.
This prohibition applies regardless of whether the sisters are full sisters, half-sisters who share the same father, or half-sisters who share the same mother.
The Rabbis enacted a safeguard to protect against violations of this prohibition, forbidding relations with the sister of one’s deceased brother’s wife in a situation of Yibum. If a married man dies without children, then his brother is required to either marry the widow (Yibum), or to release her from this bond by performing the Halisa ceremony. The Sages enacted that once a married man dies without children, his brother may not marry the deceased’s wife’s sister, even if Yibum does not occur. Even before the brother marries the widow, or even after the brother performs Halisa, he may not marry the widow’s sister as long as the widow is alive, since he has an automatic bond to the widow and she thus resembles his wife. This enactment is called “Ahot Zekukato” (the sister of the woman to whom one is bound by the Misva of Yibum) and “Ahot Halusato” (the sister of the woman to whom one performed Halisa).
This prohibition applies in all places and in all times. If one transgressed this prohibition, and had an intimate relationship with his wife’s sister or married her during his wife’s lifetime, then he is liable to Karet. If he committed the violation in the presence of witnesses who warned him in advance, then he is liable to Malkut. If the prohibition was violated unintentionally, then he must bring a Hatat (sin-offering). If he had relations with “Ahot Zekukato” or “Ahot Halusato” (as discussed above), then he receives Malkut for violating the Rabbinic edict.
If a person marries his wife’s sister in violation of this command, then this marriage does not halachically take effect. There is a rule that if a person betroths a woman with whom relations are forbidden and punishable by Karet, then the betrothal does not take effect at all. Therefore, in the case of a man who marries his wife’s sister during his wife’s lifetime, the marriage does not take effect, and he must leave the sister without needing to grant her a Halachic divorce, since the betrothal never took effect.
Many writers addressed the question of how Yaakob Abinu was permitted to marry Leah and Rachel, who were sisters. Since the patriarchs observed the Torah’s laws, how is it possible that Yaakob would be married to two sisters?
There are three classic answers to this question.
The first is the famous approach taken by the Ramban, who advanced the theory that it was primarily in Eretz Yisrael where the patriarchs observed the Misvot. The Land of Israel is where the Misvot are ideally to be observed, and thus the patriarchs, who lived before the Misvot became obligatory, observed the Misvot when they were in the land, but not necessarily during the time they spent outside the land. Yaakob married Leah and Rachel in Haran, outside Eretz Yisrael, and so he was allowed to marry two sisters. Indeed, the Ramban says, right when Yaakob returned to Eretz Yisrael, the second sister whom he had married – Rachel – passed away, such that he was not married to sisters in the Land of Israel.
The Maharal of Prague (Rav Yehuda Loew, 1520-1609) explained differently, noting that whereas we observe the Torah which was taught to Moshe and then transmitted through the generations, the patriarchs observed the Torah through Ru’ah Ha’kodesh, which is a form of prophecy. Their entire observance of the Misvot was based on prophetic inspiration, and it was through prophetic inspiration that Yaakob determined that he was to marry Rachel even after marrying Leah. Since the patriarchs to begin with observed the Misvot as revealed to them through Ru’ah Ha’kodesh, Yaakob married both Leah and Rachel because it was revealed to him through Ru’ah Ha’kodesh that with them he would produce the twelve tribes of Israel.
Finally, many scholars posited that before Leah and Rachel married Yaakob, they had the status of gentiles, and they underwent conversion to marry him. There is a famous Halachic principle that “Ger She’nitgayer Ke’katan She’nolad Dameh” – a gentile who converts is considered born anew, such that he is not considered Halachically related to his biological relatives. Hence, once Rachel and Leah converted, they were no longer considered sisters, and thus Yaakob was permitted to marry them.
Returning to the Sefer Ha’hinuch, this Misva shows us the great importance the Torah accords to peace and harmony within families. We must do everything we can to preserve the natural feelings of closeness and love that exist among family members and relatives, and to avoid tension and conflicts within the family. The Torah demands that we live joyfully and peacefully with one another – especially with our family members, with whom we are to forge especially close bonds of love and devotion.