פירוש על קידושין 129:1

If the middle one is not called “elder” or “younger” then the middle one of the second group could not be called elder. So why shouldn’t she be permitted to marry?

The Talmud solves the problem by saying that the younger group of daughters consists of only two. There is no middle daughter, for had there been one, we would have had no doubt about her being called “eldest.” And this is evident by the fact that a middle daughter is not mentioned in the mishnah.