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Why is Yitzchak mentioned first? Yishmael was older!
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Malbim: Yitzchak was Avraham's wife's son (and Yishmael, Ben ha'Shifchah). Also, Yishmael repented, and put Yitzchak in front of himself 1 (when they buried their father), unlike Esav (34). Also in the Torah, Yitzchak precedes Yishmael, but Esav precedes Yakov (Bereishis 25:9, 35:29).
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Vilna Gaon: Also according to the opinion that Yishmael did not repent, he put Yitzchak (Ben ha'Gevirtah) in front of himself (Ben ha'Amah). Esav did not do so, for also he was Ben ha'Gevirtah!
