Midrash su Levitico 18:11

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).

(Vayikra 18:11) "The nakedness of your brother's wife you shall not uncover" and (Devarim 25:5) "Her levir (her husband's brother) shall come upon her" were both stated in one pronouncement.