And the children struggled together within her; and she said: ‘If it be so, wherefore do I live?’ And she went to inquire of the LORD. And the LORD said unto her: Two nations are in thy womb, And two peoples shall be separated from thy bowels; And the one people shall be stronger than the other people; And the elder shall serve the younger. (Genesis 25:22–23)

וַיִּתְרֹצֲצוּ הַבָּנִים בְּקִרְבָּהּ, וַתֹּאמֶר אִם כֵּן לָמָּה זֶּה אָנֹכִי? וַתֵּלֶךְ לִדְרֹשׁ אֶת ה'.
וַיֹּאמֶר ה' לָהּ:
שְׁנֵי גוֹיִם בְּבִטְנֵךְ וּשְׁנֵי לְאֻמִּים מִמֵּעַיִךְ יִפָּרֵדוּ,
וּלְאֹם מִלְאֹם יֶאֱמָץ,
וְרַב יַעֲבֹד צָעִיר. (בראשית כ"ה, כב-כג)

Did Rebekah tell Isaac the prophecy she received before the twins’ birth?

The Torah says nothing, but silence cannot establish that she kept it to herself. More likely she told him eventually. If so, why did Isaac persist in choosing to bless Esau? As Nahmanides asks: “How could Isaac transgress the LORD’s word, which would not succeed?”

The answer lies in biblical prophecy’s role. Prophecy is not fortune-telling of the kind practiced by soothsayers and sorcerers. It directs proper action rather than revealing the future to satisfy curiosity. Often future prophecy is implicitly conditional upon human conduct. Thus the Torah says “If ye walk in My statutes …,” one outcome follows (Leviticus 26:3–13), while “And if ye shall reject My statutes …,” another follows (Leviticus 26:15 and onward).

Here the prophecy is deliberately enigmatic and open to different understandings. Radak’s interpretation of “the elder shall serve the younger” provides an example:

It does not mention the word “et,” which marks the object, so the matter is uncertain: it does not clarify which will serve the other, the elder the younger or the younger the elder. There is some clarification, since usually the subject comes first, except where no doubt is possible, as in “The waters wear the stones” (Job 14:19) and “The fire causeth the waters to boil” (Isaiah 64:1). [In Hebrew, the objects “stones” and “waters” come first.] The matter was not clarified in this prophecy because sometimes the elder serves the younger, as in David’s days, and sometimes the younger serves the elder, as today … (Radak on Genesis 25:23)

לא זכר עמהם מילת "את" שהיא מורה על הפָּעוּל, והנה הדבר מסופק ולא ביאר מי יעבוד את חברו, הרב את הצעיר או הצעיר את הרב. אלא שיש בו מעט ביאור, כי ברוב [המקרים] – הפועֵל הוא הראשון, אלא (=מלבד) בדברים שאין לספק בהם, כמו: "אֲבָנִים – שָׁחֲקוּ מַיִם" (איוב י"ד, יט), "מַיִם – תִּבְעֶה אֵשׁ" (ישעיהו ס"ד, א). והיה זה שלא התבאר העניין בנבואה הזאת, לפי שפעמים יעבוד הרב את הצעיר כמו שהיה בימי דוד, ופעמים יעבוד הצעיר את הרב כמו שהוא היום ... (רד"ק, בראשית כ"ה כג)

Radak observes that “וְרַב יַעֲבֹד צָעִיר,” ve-rav ya’avod tsa’ir, lacks the object marker et. It can therefore be read either as “the younger will serve the elder,” with the object first, or “the elder will serve the younger.” JPS chooses the latter. On this understanding, Rebekah’s prophecy established the brothers’ relationship – continuing struggle between the two nations descending from Jacob and Esau, “and the one people shall be stronger than the other people” – rather than Jacob’s superiority. Who stands above or below depends on their deeds. Isaac could therefore judge Esau worthy for his own reasons, while Rebekah judged Jacob the one who should be blessed.