Midrash su Levitico 18:17

13) "a woman and her mother": This tells me only of a woman and her mother. Whence do I derive (for inclusion) her daughter, the daughter of her daughter, and the daughter of her son? It is written here "zimah," and elsewhere (Vayikra 18:17) "zimah." Just as there, her daughter, her daughter's daughter, and her son's daughter, so, her.

14) Whence is it derived that males re like females (in this regard; i.e., that his progeny outside of wedlock (his daughter, his daughter's daughter and his son's daughter) are like hers (the progeny of his wife by marriage [see above])? It is written "zimah" here, and elsewhere (Vayikra 18:17) — Just as there, males are like females (in this regard), so, here. And whence is it derived that the lower (i.e., more distant kin — his father-in-law's mother and his mother-in-law's mother) are as the higher (i.e., closer kin — his mother-in-law — in this regard)? It is written here "zimah," and elsewhere (Vayikra 18:17) "zimah" — Just as there, the lower (i.e., her daughter's daughter and her son's daughter) are as the higher (i.e., her own daughter), so, here.

15) "in fire shall they be burned, he and ethhen" — (acronym of) "eth achath mehen" ("one of them, i.e., even if only one of them [wife and mother-in-law] is alive, she [evidently, the mother-in-law] is to be burned.) These are the words of R. Yishmael. R. Akiva says: he and both of them (i.e., only if both of them are alive is she [his mother-in-law] to be burned.) We have heard the punishment, but not the exhortation; it is, therefore, written (Vayikra 18:17) "The nakedness of a woman and her daughter you shall not reveal."