Reference for Yevamot 4:9
שׁוֹמֶרֶת יָבָם שֶׁקִּדֵּשׁ אָחִיו אֶת אֲחוֹתָהּ, מִשּׁוּם רַבִּי יְהוּדָה בֶן בְּתֵירָא אָמְרוּ, אוֹמְרִים לוֹ, הַמְתֵּן עַד שֶׁיַּעֲשֶׂה אָחִיךָ הַגָּדוֹל מַעֲשֶׂה. חָלַץ לָהּ אָחִיו, אוֹ כְנָסָהּ, יִכְנֹס אֶת אִשְׁתּוֹ. מֵתָה הַיְּבָמָה, יִכְנֹס אֶת אִשְׁתּוֹ. מֵת יָבָם, יוֹצִיא אֶת אִשְׁתּוֹ בְגֵט, וְאֵשֶׁת אָחִיו בַּחֲלִיצָה:
A shomereth yavam [a woman awaiting yibum] — if his brother betrothed her sister [If one of the brothers betrothed her sister after the first had fallen before them for yibum, and she were linked (for yibum) to all of them] — In the name of R. Yehudah b. Betheira it was stated: He is told: Wait [before you marry her, lest you violate "the sister of your linked one"] until your elder brother performs an act [i.e., until he either takes her in yibum or gives her chalitzah and dissolves her linkage to you.] When his brother gives her chalitzah or takes her in yibum, he may marry his betrothed. If the yevamah died, he may marry his betrothed. [For even if he had taken her in yibum and she had died, he is permitted to marry her sister.] If the yavam dies [and no other brother remained but the one who had betrothed her sister], he sends out his betrothed with a get, and his brother's wife with chalitzah. [But he does not take her in yibum by reason of "the sister of his divorcée." The gemara rules in accordance with R. Yehudah b. Betheira.]