(Genesis 26:35) "And they were a bitterness of spirit to Isaac and to Rebecca." Why is Isaac mentioned first? Because Rebecca, since she was the daughter of an idolater, was not strict about idol-worshippers, [and the conduct of Esau's idolatrous wives did not grieve her as much]. But Isaac, who was the son of holy ones and was strict about idol-worshippers, [was grieved first]; therefore Isaac is mentioned first. Another explanation: why is Isaac mentioned first? Because the matter depended upon her [Rebecca], as it is said (Genesis 25:23) "And the LORD said to her: Two nations are in your womb" [so she bore some responsibility and the grief weighed more openly on Isaac]; therefore Isaac is mentioned first.
Why Esau's Wives Grieved Isaac Before Rebecca
Yalkut Shimoni on Torah 113:2
(בראשית כו לה) וַתִּהְיֶיןָ מֹרַת רוּחַ לְיִצְחָק וּלְרִבְקָה, לָמָּה לְיִצְחָק תְּחִלָּה, אֶלָּא רִבְקָה עַל יְדֵי שֶׁהָיְתָה בַּת עוֹבֵד עֲבוֹדָה זָרָה, לֹא הָיְתָה מַקְפֶּדֶת עַל עוֹבְדֵי עֲבוֹדָה זָרָה. יִצְחָק שֶׁהָיָה בֶּן קְדוֹשִׁים, וְהָיָה מַקְפִּיד עַל עוֹבְדֵי עֲבוֹדָה זָרָה, לְכָךְ לְיִצְחָק תְּחִלָּה. דָּבָר אַחֵר מִפְּנֵי מָה לְיִצְחָק תְּחִלָּה לְפִי שֶׁהָיָה הַדָּבָר תָּלוּי בָּהּ שֶׁנֶּאֱמַר "וַיֹּאמֶר ה' לָהּ שְׁנֵי גּוֹיִם בְּבִטְנֵךְ", לְכָךְ לְיִצְחָק תְּחִלָּה.