Midrash su Levitico 20:21

8) (Vayikra 20:21) ("And a man who takes the wife of his brother, she is niddah. He has revealed the nakedness of his brother; they shall be childless.") "And a man": to exclude a minor. "who takes the wife of his brother": either by his father or by his mother. You say: either by his father or by his mother. But perhaps only by his father and not by his mother! And this would follow, viz.: He is liable here, and he is liable with his aunt. Just as his aunt by his father and not by his other, here, too, by his father and not by his mother! Let us see to what he may be compared. Do we derive his own kin from his own kin (i.e., from his sister, to whom he is forbidden — both to his sister by his father or to his sister by his mother), and this is not to be refuted by (the instance of) his aunt, who is the kin of his father — or go in this direction: We derive something (forbidden) through marriage from something (forbidden) through marriage, and this is not to be refuted by (the instance of) his sister, which is something forbidden of itself. It is, therefore, written "he has revealed the nakedness of his brother" — either by his father or by his mother.