Or HaChaim on Deuteronomy 21:15/אור החיים על דברים כ״א:ט״ו

1 והיה הבן הבכר לשניאה, "and the firstborn son is from the wife whom you hate." The Torah does not describe this as merely a possibility but assumes as a fact that the firstborn son from these two marriages will be that of the wife who is hated. It is similar to Genesis 29,31 where the Torah told us that G'd opened Leah's womb because she was hated by her husband. G'd always looks out for those who suffer from a broken heart.