Midrash su Levitico 18:13

4) (Vayikra 20:19) ("And the nakedness of the sister of your mother and the sister of your father you shall not reveal. For he has bared his kin; they shall bear their sin.") "And the nakedness of the sister of your mother and the sister of your father you shall not reveal.": "the sister of your mother": whether from her father or from her mother. "and the sister of your father": whether from her father or from her mother. — But perhaps the meaning is from her father, but not from her mother. And this would follow, viz.: Since he is forbidden to the wife of his father's brother, and he is forbidden to the sister of his mother and to the sister of his father, then just as he is forbidden (only) to the wife of his brother from his father, and not from his mother, so, he should be forbidden (only) to the sister of his mother and the sister of his mother from her father and not from her mother! It is, therefore, written (a second time, [after 18:12 and Vayikra 18:13]) "And the nakedness of the sister of your mother and the sister of your father you shall not reveal" — "the sister of your mother," whether from her father or from her mother, "and the sister of your father," whether from her father or from her mother. "For he has bared his kin": as we have stated (above, halachah 2).

5) "they shall bear their sin": We have heard the punishment but not the exhortation. It is, therefore, written (Vayikra 18:12-13): "The nakedness of the sister of your father you shall not reveal; she is the kin of your father. The nakedness of the kin of your mother you shall not reveal; for she is the kin of your mother."