אבן העזר קס״בEven HaEzer 162

Se’if 1א׳

הַכּוֹנֵס אֶת יְבִמְתּוֹ, נֶאֶסְרוּ צָרוֹתֶיהָ עָלָיו וְעַל שְׁאָר הָאַחִים, בַּעֲשֵׂה:

One who takes in his yevamah (the childless widow whom her husband's brother takes in marriage) — her co-wives become forbidden to him and to the rest of the brothers, by a positive commandment.

Se’if 2ב׳

הַחוֹלֵץ לִיבִמְתּוֹ, נֶאֶסְרָה הַחֲלוּצָה וְצָרוֹתֶיהָ עָלָיו וְעַל שְׁאָר הָאַחִים, בַּעֲשֵׂה:

When a man performs chalitza (ceremony to undo the levirate bond) on a woman bound to him for levirate marriage, that woman and her co-wives become forbidden to him and to all his brothers by positive commandment.

Se’if 3ג׳

הַחוֹלֵץ לִיבִמְתּוֹ, אָסוּר מִדְּרַבָּנָן בִּקְרוֹבוֹתֶיהָ, אֲפִלּוּ בִּשְׁנִיּוֹת. וְכֵן הִיא אֲסוּרָה בִּקְרוֹבָיו. אֲבָל הָאַחִים מֻתָּרִים בְּכָל קְרוֹבוֹתֶיהָ. וַאֲפִלּוּ לֹא חָלַץ לָהּ אֶלָּא מִסָּפֵק, אֲפִלּוּ הָכִי אָסוּר בְּכָל קְרוֹבוֹתֶיהָ (מָרְדְּכַי רֵישׁ הַחוֹלֵץ):

A man granting chalitza (separation ceremony for a woman bound for levirate marriage) to his [late] brother's wife, is Rabbinically forbidden to marry her relatives, even second degree relatives. Similarly, she is forbidden to his relatives. But his brothers are permitted to her relatives. Note: Even if he performed chalitza out of doubt, he is still forbidden to all her relatives (Mordechai, beginning of chapter haCholetz).