TT #8 · Parashas Toldos · Passuk 25:23
and Hashem answered her, “Two nations are in your womb, two separate peoples shall issue from your body; one people shall be mightier than the other, and the elder shall serve the younger.”
Torah Temimah points to Megillah 6a. E-Daf · Sefaria — Megillah 6a
“And the elder shall serve the younger” — if [the younger] merits, he will be served; if he does not merit, he will serve.
Custom (aggadah — conditional reading of the blessing)
The elder will serve the younger only if the younger merits it; otherwise the relationship is reversed.