Midrash su Levitico 20:20

6) (Vayikra 20:20) ("And a man who lies with his uncle's wife, the nakedness of his uncle he has revealed. They shall bear their sin; they shall die childless.") "And a man": to exclude a minor. "who lies with his uncle's wife": The verse speaks of the wife of his father's brother. — But perhaps it speaks of the wife of his mother's brother! It is, therefore, written (Vayikra 18:14): "The nakedness of the brother of your father you shall not reveal. To his wife do not come near"; (she is your uncle []i.e., the wife of your uncle]") It is written here (our verse) "dodathecha" (lit., "dodatho") and there "dodathecha". Just as there the verse refers to the wife of his father's brother; here, too, it refers to the wife of his father's brother. — But perhaps it refers to the wife of his father's brother by his mother, and this would follow, viz.: Just as he is forbidden to the sister of his mother and to the sister of his father, and he is forbidden to the wife of his father's brother, then just as with the sister of his mother and the sister of his father, (he is forbidden) whether (she is his sister) either by her father or by her mother, so, with the wife of his father's brother (i.e., he should be forbidden to her) whether (he is his brother) either by his father or by his mother! And, furthermore, it follows a fortiori (that he should be forbidden to the wife of his father's brother by his mother), viz.: If in an instance (levirate marriage [yibum]), he is permitted to the wife of his brother by his father by his father, he is forbidden to the wife of his brother by his mother, then in an instance where he is not permitted to the wife of his brother by his father, should it not follow that he should not be permitted to the wife of his brother by his mother! It is, therefore, written "his uncle." It is written "his uncle" here, ("the nakedness of his uncle he has revealed"), and it is written "his uncle" elsewhere ([Vayikra 25:49]) "Or his uncle or his uncle's son shall redeem him). Just as there, the family of the father (is being referred to); here, too, the family of the father (is intended).