וַיֹּ֨אמֶר יְהוָ֜ה לָ֗הּ שְׁנֵ֤י גיים [גוֹיִם֙] בְּבִטְנֵ֔ךְ וּשְׁנֵ֣י לְאֻמִּ֔ים מִמֵּעַ֖יִךְ יִפָּרֵ֑דוּ וּלְאֹם֙ מִלְאֹ֣ם יֶֽאֱמָ֔ץ וְרַ֖ב יַעֲבֹ֥ד צָעִֽיר׃
Y respondióle SEÑOR: Dos gentes hay en tu seno, Y dos pueblos serán divididos desde tus entrañas: Y el un pueblo será más fuerte que el otro pueblo, Y el mayor servirá al menor.
Rashi
ויאמר ה' לה AND THE ETERNAL SAID UNTO HER through a messenger: it was told to Shem by holy inspiration and he told it to her (Genesis Rabbah 63:7).
Chizkuni
ויאמר ה' לה, “the Lord said to her:” by using a prophet to inform her; some commentators say that seeing that Avraham was still alive. [She had turned to him and that he answered her; others say that the reason she had not told her husband about her problem was that she did not want to cause him pain when hearing about her problem, and about the fact that G-d had not revealed the answer to him.] The reason why Avraham answered her instead of her husband was why Yitzchok could not believe that Esau was a wicked person[, as she never told him about all her problems and the answer she had received].
Da'at Zekenim
ורב יעבוד צעיר, “and the older one will serve the younger one.” Our author understands the word רב here not as “older, senior,” as do most commentators, but as meaning הרבה, “many, a great number.” Esau himself used the word in that sense when he told his brother Yaakov: יש לי רב, “I have lots,” (Genesis 33,9) The word may mean “lots” in terms of “time;” i.e. Esau would serve Yaakov for a long time.