Reference for Yevamot 2:6
מִי שֶׁקִּדֵּשׁ אַחַת מִשְּׁתֵּי אֲחָיוֹת וְאֵינוֹ יוֹדֵעַ אֵיזוֹ מֵהֶן קִדֵּשׁ, נוֹתֵן גֵּט לָזוֹ וְגֵט לָזוֹ. מֵת, וְלוֹ אָח אֶחָד, חוֹלֵץ לִשְׁתֵּיהֶן. הָיוּ לוֹ שְׁנַיִם, אֶחָד חוֹלֵץ וְאֶחָד מְיַבֵּם. קָדְמוּ וְכָנְסוּ, אֵין מוֹצִיאִין מִיָּדָם:
If a man betrothed one of two sisters and did not know which one, [so that both are forbidden to him, each one possibly being the sister of the woman he married], he gives a get to each. If he died and had another brother, he gives chalitzah to both [for he does not know which is his yevamah. And he cannot give chalitzah to one and take the other in yibum, by reason of "the sister of his chalutzah." (And she is forbidden to him in her sister's lifetime by rabbinical ordinance.) Nor can he take one in yibum first, for she might not be his yevamah, in which instance he would be violating the sister of one "linked" to him, who is like his wife. If he had two brothers, one gives chalitzah [to one first], and the second, [if he wishes,] takes [the other] in yibum. [For if she (the other) is his yevamah, all well and good; and if she is the yevamah's sister, he does not violate "the sister of his linked one," for the chalitzah of the other brother had dissolved the linkage.] If the brothers were beforehand and married them [before taking counsel with beth-din], they are not taken from them [for each one can say: "I married correctly." And even if it were otherwise — that the first who wed for yibum really took the sister of his linked one — it was at that time that he transgressed, and when his brother came and took the other, the true yevamah, in yibum, the linkage to the other was retroactively dissolved.]