כִּֽי־תִהְיֶ֨יןָ לְאִ֜ישׁ שְׁתֵּ֣י נָשִׁ֗ים הָאַחַ֤ת אֲהוּבָה֙ וְהָאַחַ֣ת שְׂנוּאָ֔ה וְיָֽלְדוּ־ל֣וֹ בָנִ֔ים הָאֲהוּבָ֖ה וְהַשְּׂנוּאָ֑ה וְהָיָ֛ה הַבֵּ֥ן הַבְּכ֖וֹר לַשְּׂנִיאָֽה׃
어떤 사람이 두 아내를 두었는데 하나는 사랑을 받고 하나는 미움을 받다가 그 사랑을 받는 자와 미움을 받는 자가 둘 다 아들을 낳았다 하자 그 미움을 받는 자의 소생이 장자여든
Sforno on Deuteronomy
כי תהיינה לאיש שתי נשים, after the victory in war the Torah addresses internal matters in the Jewish state. The most important ones are interpersonal relationships with wives, children, livestock, jewelry, hunting for food, building homes, tilling the soil and matters related to clothing.
Or HaChaim on Deuteronomy
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.
Tur HaArokh
כי תהיין לאיש שתי נשים, האחת אהובה, “When a man has two wives, one of whom he loves, etc.” This does not mean that he actually hates one of his wives, but it is inevitable that when one has two wives that one must love one better that the other, and the other is described as the one that “he hates.” Each of the two wives (in any marriage) employs devious means including sorcery to become her husband’s favourite.