Kiddushin 65A

אמצעית שבכת שניה תשתרי! הכא במאי עסקינן, בשאין שם אלא גדולה וקטנה.

with regard to one who said: I betrothed my elder daughter, the middle daughter of the younger second group should be permitted, as he would have called her by name rather than referring to her as: The elder one. The Talmud answers: Here we are dealing with a case where there are only two daughters, an adult woman and a minor girl, but no middle daughter.

והכי נמי מסתברא, דאם איתא דאיכא, ליתנייה. ולטעמיך, אמצעית שבכת ראשונה, דודאי ספיקא ואסירא ליה, מי קתני לה?

The Talmud adds: And so too, it is reasonable that this is the case, as, if it is so, that there is a middle daughter, let the Mishnah teach its halakha with a direct reference to her as well, as the uncertainty also applies to this daughter. In other words, the Mishnah should have stated: And I do not know if it was the middle of the younger group of daughters. The fact that the Mishnah does not refer to this daughter indicates that there are only two women in each group. The Talmud rejects this suggestion: But according to your reasoning, the middle one of the first group is definitely included in the uncertainty and is forbidden to the prospective husband, and yet does the Mishnah teach its halakha with a direct reference to her?

הכי השתא?! התם תנא קטנה דידה לאיסורא, והוא הדין להך דקשישא מינה,

The Talmud questions this argument: How can these cases be compared? There it taught its halakha with a direct reference to a daughter who is younger than the middle daughter of the older group, and that daughter is mentioned for a prohibition, as the Mishnah states that the uncertainty applies even to the youngest of the older group; and if so, the same is true of this middle daughter, who is older than the youngest of the older group, i.e., it is evident that the same uncertainty applies to her, and therefore there is no reason to mention the middle daughter of the older group.