Leviticus 20:14

ישרפו אתו ואתהן [AND IF A MAN TAKE A WIFE AND HER MOTHER, IT IS WICKEDNESS] THEY SHALL BE BURNT [WITH FIRE] BOTH HE AND THEY — You cannot say that this means that they shall burn also his first wife (i. e. her whom he married first), because you must admit that he married her legally for she was not forbidden to him as a wife. But the fact is that the case of “a woman and her mother": mentioned here must be a case where both of them are forbidden to him on account of a previous relationship to him, — that, for instance, after having married a woman, he marries also his mother-in-law and her mother. It is these two that have to be burnt. There are, however, some of our Rabbis who say we have here a case of where, in addition to having married a woman, he marries only his mother-in-law (but not also the latter's mother). But what then could be the meaning of אתהן? It means one of them (“את "הן) and this is a Greek word, הן (ξυ) meaning “one" (Sanhedrin 76b; Sifra, Kedoshim, Chapter 10 12).

וְאִ֗ישׁ אֲשֶׁ֨ר יִקַּ֧ח אֶת־אִשָּׁ֛ה וְאֶת־אִמָּ֖הּ זִמָּ֣ה הִ֑וא בָּאֵ֞שׁ יִשְׂרְפ֤וּ אֹתוֹ֙ וְאֶתְהֶ֔ן וְלֹא־תִהְיֶ֥ה זִמָּ֖ה בְּתוֹכְכֶֽם׃

And if a man take a wife and her mother, it is wickedness: they shall be burnt with fire, both he and they; that there be no wickedness among you.