פירוש על קידושין 128:6

When a man says “I betrothed my daughter” without specifying which, he cannot be referring to his daughter who is a bogeret, but he can be referring to his minor daughter. But this implies that all of his minor daughters are impacted by his statement, even though, if he has more than one, this is a case of “kiddushin that cannot be followed by intercourse” because any one girl could be his wife’s sister. Thus we could learn that in such cases the kiddushin is at least doubtfully valid—each minor daughter is doubtfully betrothed. To recall, this was a major topic discussed earlier in the tractate and thus we do not want this mishnah to answer the question.
The Talmud answers that there was only one minor. Thus there is no doubt that she is the one who is betrothed.