Midrasch zu Wajikra 18:9

8) (Vayikra 18:3) "and in their statutes you shall not walk": What did Scripture leave unsaid (that this need be stated)? Is it not already written (Devarim 18:10) "There shall not be found among you one who passes his son or daughter through fire … (Devarim 18:11) and a chover chaver, etc."? What, then, is the intent of "and in their statutes you shall not walk"? In their customs — those things that are established for them — such as theatres, circuses, and sports. R. Meir says: These are "the ways of the Emorites," which the sages enumerated. R. Yehudah b. Betheira says: that you not preen yourself (to attract women), and not cultivate locks, and not wear the hair komi (a gentile fashion). And lest you say: "They have statutes and we have no statutes" — It is, therefore, written (Vayikra 18:9) "My judgments shall you do and My statutes you shall heed to walk in them; I am the L–rd your G d." — But there is still "hope" for the yetzer hara to reflect and say "But theirs are more beautiful than ours!" — It is, therefore, written (Devarim 4:6) "And you shall heed and you shall do. For this is your wisdom and your understanding."

12) "And they shall be cut off before the eyes of their people." We have heard the punishment, but we have not heard the exhortation. It is, therefore, written (Vayikra 18:9) "The nakedness of your sister, the daughter of your father or the daughter of your mother … you shall not reveal." This tells me only of ht 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 ) "your sister" (Vayikra 18:11) — in any event. — But even without the repetition, I could derive it a fortiori, viz.: If he is exhorted 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 exhorted for his sister) both from his father and from his mother! — But if you say this, you have exhorted by means of an a fortiori argument — wherefore it is written "your sister," to teach that there is no exhortation by means of an a fortiori argument.

13) If (Vayikra 18:9) "your sister," I might think even by a maidservant or by a gentile woman; it is, therefore, written (Vayikra 18:11) "the daughter of your father's wife." If "the daughter of your father's wife," I would think even (the daughter) by another man; it is, therefore, written "begotten of your father" If so, why is it written (Vayikra 18:9) "born of the home or born of the outside? Whether (born of one concerning whom) we tell him "Keep her" (in the home, i.e., one that he was permitted to marry) or (born of one concerning whom) we tell him "Send her out" (i.e., one that he was not permitted to marry).