The sister of the mother, from where [is this derived]? As it is stated: “The nakedness of your mother’s sister you shall not uncover; she is your mother’s near kinswoman.” And it is taught [in Yevamot 54]: I only know the sister of his father from the father’s side and the sister of his mother from the father’s side, as the family of the father is called family. From where do we derive the sister of his father from the mother’s side and the sister of his mother from the mother’s side? The Torah states in the section of Kedoshim, “The nakedness of your mother’s sister and your father’s sister you shall not uncover,” to warn us that even [if they are] from the mother’s side. And their punishment is excision, as stated in the section of Acharei Mot at the end of all the sexual prohibitions: “And the souls that perform [these acts] shall be cut off.”