The basic concept of yibum is presented in a short set of pesukim in Sefer Devarim (25:5-10). The very essence of this mitzva is unusual. Generally speaking, a man is forbidden from marrying his brother’s wife, a prohibition listed among the sexual relationships that are forbidden by Torah Law that carry with them the severe punishment of karet. In this specific case, the Torah is clear: the commandment to perform yibum eliminates the prohibition to marry one’s sister-in-law. Is that true even if the relationship is forbidden for other reasons, as well?
The tradition that the Sages present in the Mishna is clear. Only the prohibition to marry one’s brother’s wife is removed; all other existing prohibitions remain in place and will keep the mitzva of yibum from being fulfilled.
Massekhet Yevamot opens on our daf with a list of 15 women who will not “fall” to yibum because of their relationship with the surviving brother (examples are when the widow is his daughter, niece, or mother-in-law). The Mishna teaches that this is true not only of these women, but also of their rival wives (referred to in the Mishna as tzarot) in the event that the husband who passed away was married to more than one woman. It is important to remember that polygamy is permitted by the Torah and is not practiced today largely because of a tradition dating from the 11th century CE.
One of the questions dealt with by the rishonim is why Yevamot was chosen to be the first Mishnayot in Seder Nashim, when it appears to deal with a relatively obscure case of marriage. Either Massekhet Ketubot or Massekhet Kiddushin, which deal with basic issues of marriage, would seem to be a more logical choice. The Rambam suggests that most cases of marriage are voluntary, while yibum is a positive commandment fulfilled by the yavam, so it was deemed appropriate to be placed first. The Rosh offers two reasons. First of all, a careful examination of the laws of yibum also offers a clarification of forbidden sexual relationships. These rules logically precede the laws of marriage. Also, according to the Rosh’s order of the Mishna, the last set of Mishnayot in Seder Mo’ed was Massekhet Moed Katan, which closed with the laws of mourning. Those laws naturally flow into a discussion of yibum, which occurs when a tragic death strikes a family.