Midrash su Levitico 20:17

10) (Vayikra 20:17) ("And a man who takes his sister, the daughter of his father or the daughter of his mother, and sees her nakedness, and she sees his nakedness, it is chesed. And they shall be cut off before the eyes of their people. The nakedness of his sister he has revealed; he shall bear his sin.") "And a man": to exclude a minor. "who takes his sister, the daughter of his father or the daughter of his mother": This tells me only of the daughter of his father, who is not the daughter of his mother, or the daughter of his mother, who is not the daughter of his father. Whence do I derive (for inclusion his sister) both from his father and from his mother? From (the repetition of) "his sister" — in any event. — But even without the repetition, I would derive it a fortiori, viz.: If he is liable for his sister from his father, who is not the daughter of his mother, and from his mother, who is not the daughter of his father, how much more so (is he liable for his sister) both from his father and from his mother! — But if you say this, you have punished by means of an a fortiori argument — wherefore it is written "his sister," to teach that there is no punishment by means of an a fortiori argument.

Rabbi Miasha said: Cain was born, and his wife, his twin-sister, with him. Rabbi Simeon said to him: Has it not already been said, "And if a man shall take his sister, his father's daughter, or his mother's daughter, and see her nakedness, and she see his nakedness; it is a shameful thing"? (Lev. 20:17). From these words know that there were no other women whom they could marry, and these were permitted to them, as it is said, "For I have said, The world shall be built up by love" (Ps. 89:2). With love was the world built up before the Torah had been given. Rabbi Joseph said: Cain and Abel were twins, as it is said, "And she conceived, and bare (with) Cain" (Gen. 4:1). At that hour she had an additional capacity for child-bearing (as it is said), "And she continued to bear his brother Abel" (Gen. 4:2).