Yevamot 2 - Sister-in-law
In general, a man is prohibited to marry his sister-in-law, that is, the wife of his brother. However, if his brother dies childless, then it is a mitzvah to marry her (and this is called a "yibum") or to formally allow her to go free (and this is called a "chalitzah").
A special situation arises in a case where a man cannot marry her for a different reason, such as if she is the man's daughter. And, since he cannot marry his daughter, and she does not even need a chalitzah, so too the other wives of the deceased brother are released without a yibum or chalitzah.
Two things require explanation. How can a man's brother be married to his daughter? Should not such a marriage be prohibited? The answer is that there is no prohibition in the Torah to marry one's niece, but one can't marry his aunt. What about the other wives (co-wives), how is that allowed? The answer here is that by Torah law a man is allowed many wives. The Talmud goes on to list fifteen close relatives of a man whom he cannot marry and who are thus automatically released from chalitzah or yibum, together with their co-wives.
Practically, today one cannot marry many wives and yibum is not allowed, but the chalitzah is still required.
Art: The Two Sisters by August Andreas Jerndorff