Yevamot, Chapter One, Mishnah Three
Mishnah three introduces a different list of women which is compared to the list in mishnah one.
There are six relatives that are more restricted than these, in that they may be married only to strangers, marriage with their rivals is permitted: his mother and his fathers wife, his fathers sister, his paternal sister, his fathers brothers wife and his paternal brothers wife.
The six women mentioned in this mishnah are considered more restricted because they cannot be married to a mans paternal brother (remember, yibbum is restricted to only paternal brothers). For instance, a mans mother cannot marry his paternal brother, because she is the wife of the paternal brothers father, and it is forbidden to marry your fathers wife. Likewise, it is forbidden to marry ones fathers sister, ones sister (from either parent), ones fathers brothers wife or ones brothers wife. This last case can occur when one brother marries a woman and divorces her or dies with children. This wife is now forbidden to all of her dead or divorced husbands brothers.
In all of these cases, if one of these women was married to another man and their husband died, their rival wives may marry one of the brothers. The rival wife of a woman forbidden due to kinship is forbidden only when the forbidden women became eligible for yibbum. These women were never eligible for yibbum to these brothers, therefore their rival wives are permitted. Furthermore, even if the brother tried to marry one of these women, and then died, the rival wives are eligible for yibbum, since the marriage of the brother to the forbidden woman was not valid.