Brotherly Kindness

The Torah says that if a man has relations with his sister, it is a chesed, and he shall be cut off [i.e. executed] in front of the nation[1]. The word chesed typically means "kindness"; however, in this instance, Rashi explains that the word is related to chaisuda, which means "disgrace" in Aramaic[2]. Nachmanides finds[3] difficulty in assuming that the word chesed in this context means something very different from the usage of the word throughout the rest of the Torah. Therefore, Nachmanides explains that even in this situation, the word chesed means "kindness." He explains that the Torah was saying that when a man marries his sister, he is liable for punishment because he should have acted kindly with her by not marrying her because inter-sibling marriages are always destined to fail, and the man was obviously acting of purely selfishness, which is the antithesis to kindness. The Talmud asks[4] why Eve did not die immediately after she ate from the Forbidden Fruit, as punishment for her horrible sin. In asking this question, the Talmud says that Eve's role of companion to her husband Adam could easily have been taken over by Adam and Eve's daughter, whom Adam could have married if Eve died. The Talmud answers that had Adam married his own daughter, Cain would not have been able to marry her[5], and so the world's population would never have continued. Since Eve was not killed for her sin with the Forbidden Fruit, Cain was therefore able to marry his sister[6] and continue the progeny of the world. The Talmud proves this idea by quoting the verse which states, "I said, 'the world was created through kindness'.[7]" The kindness that created the world was the kindness of HaShem by not punishing immediately Eve with death for eating from the Forbidden Fruit. Part of this kindness was allowing Cain to marry his sister and continue the population of the world. From here, Nachmanides understands how marrying one's sister can be called a kindness.

According to Nachmanides, it is only forbidden for a man to marry his sister if it is otherwise possible for the man to do a kindness to her by finding her a suitable match. However, in the case of Cain and his sister, no suitable match existed, as they were the only people in the world, so Cain was legally able to marry his own sister. Rabbi Mordechai Kornfeld says[8] that perhaps this explains how the sons of Jacob were able to marry their own twin sisters. In theory, Jacob would have allowed his daughters to marry foreign men, as long as they converted to the Hebrew religion and accepted certain tenants of belief. However, he saw that all the gentiles wished to remain idol worshippers, and did not want to accept upon themselves the yoke of heaven[9]. Since there were no suitable matches for Jacob's daughters because everyone, save from the Abrahamic family, was idolaters, the sons of Jacob performed a kindness to their sisters by marrying them.

[1] Leviticus 20:17
[2] See Targum Onkelos ibid. and to Genesis 34:4
[3] Ramban to Leviticus 20:17
[4] Sanhedrin 58b
[5] Because she would have been his father's wife
[6] Her name is given in various sources as Kalmana (see Seder HaDoros, and Abrabanel to Genesis 4:1), Lebuda, and Awan (in the Christian New Testament Book of Jubilees)
[7] Psalms 89:3
[8] Kedoshim, 5757
[9] Ra'avad to Avodah Zarah 36b