Yevamos, Chapter Two, Mishnah 8.
"Mitzvah b'gadol l'yabem":
The Mishnah opens: "Mitzvah b'gadol l'yabem" - the mitzvah of yibum rests upon the oldest brother. The source of this law is the verse "And it shall be that the firstborn whom she bears." The plain meaning of the verse is that the firstborn son who will be born should be called by the name of the deceased, but our Sages expounded the verse and derived from it that it is the oldest of the brothers - the one born first in the family - upon whom the obligation to fulfill the mitzvah of yibum rests.
"Kadam hakatan - zachah" - and if the younger brother preceded him and performed yibum first, he has acquired the mitzvah and fulfilled it. Even so, the primary obligation rests upon the oldest.
One suspected with a maidservant or with a non-Jewish woman:
Two cases are taught here, in which a man was suspected of having relations with a woman forbidden to him, and afterwards she became permitted to him:
"Hanit'an al hashifchah v'nishtachrerah" - a man who was suspected of having relations with a Canaanite maidservant, who is forbidden to a Jew, and afterwards she was freed. Her release makes her a full convert, and from then on she is permitted to a Jew.
"O al hanochris v'nisgayrah" - and likewise one who was suspected of having relations with a non-Jewish woman, and afterwards she converted and entered beneath the wings of the Divine Presence.
In both cases the Mishnah rules: "Harei zeh lo yichnos" - he may not marry her in the first instance, as it says in Mishlei, "Remove from yourself crookedness of the lips," and a person must distance himself from the talk of those who gossip about him. For the matter appears as though she was freed or converted only on account of the illicit relationship between her and that Jew, so that she would be able to marry him.
"Uv'diavad ein motzi'in miyado" - however, if he already married her, they are not compelled to divorce, and they are permitted to remain married. The prohibition applies only in the first instance.
"Hanit'an al eishes ish":
One who was suspected of a relationship with a married woman, and the suspicion was so weighty that "they took her away from him" - that is, they compelled her husband to divorce her, because the evidence was strong enough to require a divorce - in such a case, even if the suspected man afterwards married the woman, "yotzi": he is obligated to divorce her. For once the suspicion reached a level at which it is clear that there truly was a relationship between them, a woman who committed adultery is forbidden to her husband and is also forbidden to the man with whom she sinned - and therefore even if he married her, he must divorce her.
In summary: In this Mishnah we learned that the mitzvah of yibum rests in the first instance upon the oldest brother, and if the younger brother preceded him and performed yibum, he has acquired the mitzvah. We also learned the law of one suspected with a maidservant who was freed or with a non-Jewish woman who converted: in the first instance he may not marry her because of "crookedness of the lips," but after the fact they are not compelled to divorce. By contrast, one suspected with a married woman who was taken away from her husband - if he married her, he must divorce her.