Midrash su Levitico 20:19

2) He has bared (he'erah) her fountain, and she has revealed the fountain of her blood" — whence it is derived that peripheral (non-consummated) intercourse ("he'arah") is reckoned as consummated intercourse. This tells me only of niddah. Whence do I derive the same for all of the arayoth? — Just as with niddah, where willful transgression is liable to kareth and unwitting transgression, to a sin-offering, he'arah is reckoned as consummated intercourse — so, with every ervah, where willful transgression is liable to kareth and unwitting transgression, to a sin-offering, he'arah is reckoned as consummated intercourse. — No, this may be so with niddah, where tumah obtains, wherefore he'arah is reckoned as consummated intercourse, but not with all the other arayoth, where tumah does not obtain, wherefore he'arah is not reckoned as consummated intercourse! It is, therefore, written (in relation to the sister of his father or of his mother) "For he has bared (he'erah) his kin." You derive it from both, viz.: Niddah (where tumah obtains) is not like the sister of his father, and the sister of his father, (who is kin), is not like niddah. What is common to both is that they are ervah, where willful transgression is liable to kareth, and unwitting transgression to a sin-offering, and he'arah is reckoned as consummated intercourse. Likewise, I derive that with every ervah, where willful transgression is liable to kareth and unwitting transgression to a sin-offering, he'arah is reckoned as consummated intercourse.

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).