Sibling love, disgrace, and quarrels
ואיש אשר-יקח את-אחתו בת-אביו או בת-אמו וראה את-ערותה והיא תראה את ערותו חסד הוא ונכרתו לעיני בני עמם ערות אחתו גלה עונו ישא
A man who will take his sister, the daughter of his father or the daughter of his mother, and will see her nakedness, and she will see his nakedness, it is chesed, and they will be cut off from before the eyes of the nation. For he has uncovered his sister’s nakedness, and he shall carry his sin
The Torah, in its list of the forbidden relationships, gives the incestuous relationship with one’s sister a special descriptor. The union is referred to as chesed. Normally, this word refers to loving kindness. It seems highly out of place in this context. Rashi therefore says that in this context it’s the Aramaic word for disgrace. Such a union is a disgrace to both parties. However, why did the Torah use this unusual word, instead of the normal Hebrew word for disgrace? Rashi therefore brings the homiletic interpretation, that this verse is alluding to the answer to an age-old question.
Hashem created Adam and Eve, the first humans. Together they had two sons, Kayin and Hevel. Chazal inform us that Kayin was born together with a sister, and Hevel with two sisters. To ensure the perpetuation of mankind, Kayin and Hevel married their twin sisters. If incest with one’s sister is forbidden, how could Kayin and Hevel marry their sisters? The answer is that Hashem performed chesed for humanity and temporarily permitted this union. Hashem’s act of loving kindness ensured the continuity of mankind. To allude to this fact, the Torah uses the word chesed when it lists the forbidden union of a man and his sister.
However, this solution to perpetuating humanity seems unnecessary. Wouldn’t it have been simpler to create multiple families of humans? They could each marry each other’s children, avoiding any incest whatsoever. Why did Hashem create a single Man, to then divide him into husband and wife, such that their kids would have to marry each other? This approach requires temporarily lifting the prohibition on marrying one’s sister. The answer is that Hashem created Man alone to teach us the importance and significance of a single human being. When someone saves the life an individual, it’s as if they have saved an entire world, and when they destroy a life, it’s as if they have destroyed an entire world. As well, it avoids unnecessary quarreling about who has a greater ancestor. We all come from the same source. It was a tremendous chessed then that Hashem created mankind in a way that He’d have to permit a brother to marry his sister, just to teach us this lesson, and to maintain peace.
With this, we can also get a deeper understanding of Hashem’s rebuke of Kayin after he killed his brother Hevel. Hashem told Kayin: “Behold, the voice of your brother’s blood calls out to Me”. Rashi explains that this refers to the blood of Hevel, as well as all his potential descendants. Why did Hashem introduce his rebuke with “behold”? It sounds like He’s trying to point something out that Kayin wasn’t noticing.
Why did Kayin kill his brother Hevel? The verse is very vague. It just says that they went out into the field, and they had a discussion about something. What was it about? Chazal interpret that they were arguing about their twin sisters. Kayin was telling his brother Hevel that Hashem abhors incestuous relationships. Despite that knowledge, they were in an impossible situation. The only way to continue the human race was to marry their sisters. Kayin suggested to Hevel that to decrease the impropriety as much as possible, Hevel should take the sister that was born with Kayin, and Kayin should take the sisters who were born with Hevel. Hevel however wasn’t interested in this proposition. As a result, Kayin killed his brother Hevel.
Hashem’s rebuke to Kayin was to point out his hypocrisy. If a man marrying his sister is so abhorrent to Hashem, why did He create the world in such a way that it was the only way to perpetuate the human race? The only reason is as said before, to teach us that a single person is an entire world, and someone who destroys a life is as if they have destroyed an entire world. If so, why did Kayin kill his own brother? He had no answer. Hashem told him that the blood of his brother Hevel was crying out. This was his blood and the blood of all his potential offspring. This single person was like an entire world.
 Based on Chasam Sofer’s Toras Moshe I to Leviticus 20:17
 Leviticus loc. cit.
 Cf. Radak’s Shorashim § חסד. See also Pri Tzaddik I Kedushas Shabbos § 4 and Michtav MeEliyahu II p. 164-165, who explain that a perversion of chesed can lead to sexual immorality, as seen by Yishmael, who took his father Avraham’s pillar of chesed to an inappropriate extreme (see Kiddushin 49b)
 Ad. loc.
 Sanhedrin 58b; Toras Kohanim ad. loc. § 11
 Genesis 4:1-2
 Bereishis Rabbah 22:3, brought by Rashi ad. loc.
 Cf. Pirkei D’Rabbi Eliezer Chapter 21, that Hevel also only had one twin sister. See Torah Sheleimah ad. loc. § 8
 This is based on Psalms 89:3
 See Genesis 1:27 with Rashi, quoting Eruvin 18a and Bereishis Rabbah 8:1
 Sanhedrin 4:5
 Genesis 4:10
 Literally: bloods. See next note
 Ad. loc., quoting Bereishis Rabbah 22:9
 The Chasam Sofer just asks why Hashem said הנה, and his answer doesn’t clearly address this. Therefore, this is what I think he’s really asking
 Genesis 4:8
 Sotah 9b with Rashi s.v. קין and Tosafos Shantz s.v. וכן מצינו בקין; Bereishis Rabbah 22:7; Pirkei D’Rabbi Eliezer loc. cit.; Zohar I parshas Bereishis p. 36b, 54b; Tikkunei Zohar p. 113b. The Chasam Sofer brings the justification for this interpretation from the fact that the verse says בהיותם בשדה, and in the parsha of the betrothed maiden (Deuteronomy 22:17) it says בשדה מצאה. This connection is made by the Zohar and Tikkunei Zohar. Pirkei D’Rabbi Eliezer connects the בשדה of the verse to כי האדם עץ השדה (Deuteronomy 20:19). Bereishis Rabbah doesn’t bring a justification, but Maharzu and Beur MehaRif ad. loc. suggest the same as Pirkei D’Rabbi Eliezer
 The above sources only say that Kayin wanted to marry Hevel’s extra twin sister. The Chasam Sofer even brings it this way, but then gives his own original interpretation of the conflict, proposing a sort-of swap