Rather, is it not referring to a case where he said to them: One of you is betrothed, and it teaches that the sisters are not betrothed, because with regard to the sisters the betrothal is not given to consummation? The Talmud comments: If so, the first clause of the Mishnah is difficult for Rava, while the last clause is difficult for Abaye. The Talmud presents a resolution: Abaye explains the last clause according to his line of reasoning, and Rava explains the first clause according to his line of reasoning.
אביי מתרץ לטעמיה: המקדש אשה ובתה, או אשה ואחותה כאחת – אינן מקודשות. הא אחת מאשה ובתה, מאשה ואחותה – מקודשת.
Abaye explains it according to his line of reasoning, as follows: In the case of one who betroths a woman and her daughter or a woman and her sister in one act of betrothal neither of them is betrothed. But if he betrothed one of a pair consisting of a woman and her daughter or a woman and her sister by saying: One of you is betrothed to me, each of them is betrothed as a matter of uncertainty and requires a bill of divorce, despite the fact that the betrothal is not given to consummation, in case he is in fact betrothed to the other one.
ואם אמר: ״הראויה לביאה תתקדש לי״ – אינה מקודשת. ומעשה נמי בחמש נשים ובהן שתי אחיות, וליקט אחד כלכלה של תאנים ואמר: ״הראויה לי מכם תתקדש לי״, ואמרו חכמים: אין אחיות מקודשות.
The Talmud continues Abaye’s explanation: And if he explicitly said: The one of you fit for sex should be betrothed to me, neither of them is betrothed, as he cannot have sex with either, not knowing which of them is his betrothed and which is his betrothed’s relative. And an incident also occurred involving 5 women, and among them were two sisters. And one man gathered a basket of figs and said: The one among you fit for sex with me should be betrothed to me with this basket, and the rabbis said: The sisters are not betrothed.