Kiddushin, Chapter Three, Mishnah Nine
This mishnah continues to discuss a father who betroths one of his daughters but does not remember which one he betrothed. Specifically, the mishnah refers to a situation where a man has two sets of daughters with two different wives and one set of daughters is older than the other set (i.e. all of the daughters of one wife are older than the eldest daughter of the other wives.)
1) If one has two groups of daughters by two wives, and he declares, I have given in betrothal my eldest daughter, but I do not know whether the eldest of the seniors or the eldest of the juniors, or the youngest of the seniors who is older than the eldest of the juniors, all are forbidden [to marry other men], except the youngest of the juniors, the words of Rabbi Meir.
a) Rabbi Yose says: they are all permitted, except the eldest of the seniors.
2) I have betrothed my youngest daughter, but I do not know whether the youngest of the juniors or the youngest of the seniors, or the eldest of the juniors who is younger than the youngest of the seniors, they are all forbidden, except the eldest of the seniors, the words of Rabbi Meir.
a) Rabbi Yose says: they are all permitted, except the youngest of the juniors.
Section one: The father says he has given in betrothal his eldest daughter to a man, and he has forgotten who that man is, and the question is who his eldest daughter is? Once we figure out whom the eldest daughter is that daughter will be forbidden to marry any other man, and his other daughters will be permitted. Again, the man has two sets of daughters; one set is a group of seniors in that they are all older than the other set which are juniors. According to Rabbi Meir, any daughter who has a younger sister may be called the eldest daughter since she is older than another daughter. The only daughter who cannot possibly be called the eldest daughter is the youngest daughter of the youngest set.
Rabbi Yose rules that only the eldest daughter of the seniors is called the eldest daughter and therefore only she is prohibited.
Section two: This section is basically the same as the previous section, except that in this section the father claims that he gave his youngest daughter in betrothal. The opinions of Rabbi Meir and Rabbi Yose are consistent with those above.