Before us is Mishnah 6 in the third chapter of Maseches Yevamos. The Mishnah deals with three brothers, two of them married to two sisters and the third married to an unrelated woman, and it examines two situations that differ from one another in the order in which the brothers died.
The first case - one of the husbands of the sisters died:
"Sheloshah achim, shenayim mehem nesu'im shtei achayos ve'echad nasui nochris" - two of the brothers are married to two sisters, whom we will call Rachel and Leah; the third brother, whom we will call Levi, is married to a woman who is not a relative at all.
"Meis echad mibaalei ha'achayos" - the brother married to Leah died.
"Vechanas nasui nochris es ishto" - Levi performed yibum with Leah, while his first wife, the unrelated woman, remained in his home.
"Umeis" - and afterwards Levi himself died, and two women now fall before the remaining brother.
And these are their laws:
The first (Leah): she goes free as a wife's sister - she is the sister of the remaining brother's wife, forbidden to him as an ervah, and therefore she neither undergoes yibum nor performs chalitzah.
The second (the unrelated woman): she goes free as her co-wife - she is the co-wife of an ervah, and she too is exempt from chalitzah and from yibum.
"Asah bah maamar umeis":
What is the law if Levi did not actually perform yibum with Leah, but only made a maamar with her - a bond of betrothal that is only rabbinic - and then died? Leah herself certainly does not undergo yibum with the remaining brother, since she is his wife's sister and forbidden to him as an ervah. But the unrelated woman is not a full co-wife of an ervah, because the two of them did not truly become co-wives except by force of a maamar alone.
The Mishnah therefore rules: "Nochris choletzes velo misyabemes" - she does not undergo yibum, because she appears to be the co-wife of an ervah on account of the maamar that was made with Leah, who is a wife's sister; on the other hand, she cannot be released entirely with nothing, since by Torah law she is still bound to yibum, and therefore she performs chalitzah.
The second case - the one married to the unrelated woman died:
"Sheloshah achim, shenayim mehem nesu'im shtei achayos ve'echad nasui nochris" - the very same situation, except that the order of their deaths is different.
"Meis nasui nochris" - the first to die is Levi, who is married to the woman who is not a relative.
"Vechanas echad mibaalei achayos es ishto" - one of the brothers married to the sisters performed yibum with the unrelated woman.
"Umeis" - and afterwards he too died, and his two wives fall before the remaining brother: his first wife, who is one of the sisters, and the unrelated woman with whom he performed yibum.
And these are their laws:
The first (one of the sisters): she goes free as a wife's sister - with regard to the remaining brother she is his wife's sister, forbidden to him as an ervah and exempt.
The second (the unrelated woman): she goes free as her co-wife - once he performed yibum with her, the two became co-wives of one another, and she is thus the co-wife of an ervah.
"Asah bah maamar umeis":
And if the brother married to one of the sisters did not perform yibum with the unrelated woman, but only made a maamar with her - betrothal by rabbinic law - and then died: his wife, the sister, goes free as a wife's sister, while the unrelated woman "choletzes velo misyabemes". She does not undergo yibum, since she appears to be the co-wife of an ervah; and since there is no full co-wife of an ervah here, only by force of the maamar, she cannot be released entirely, but rather she performs chalitzah.
In summary: in both cases in the Mishnah two women fall before the remaining brother, and when a full yibum was performed both are exempt - one as a wife's sister and the other as her co-wife. By contrast, when only a maamar was made, which is nothing more than rabbinic, there is no full co-wife of an ervah here, and therefore the unrelated woman performs chalitzah and does not undergo yibum.