The Son of the Hated Wife is Always the Firstborn

The Son of the Hated Wife is Always the Firstborn:

The Abarbanel notes that the pasuk doesn't say "if the first son is born to the hated wife"; rather, it says that he will be born to her, as a statement of fact. He explains that if a man has one wife that he loves and one that he hates, the firstborn son will always come from the hated wife, as Hashem will have mercy on her just like he had on Leah, whom he granted a firstborn son because she was "hated" (Bereishis 29:31). This is in accordance with the dictum that "Hashem takes care of the oppressed (Koheles 3:15)."