"...in disregard of the son of the unloved one who is older": Although the older is of the unloved one.
(Devarim 21:17) "But the first-born, the son of the hated one, shall he recognize": He shall cause him to be recognized by others — whence R. Yehudah ruled: A man is believed to say: This is my first-born son. And just as he is believed to say: This is my first-born son, he is believed to say: This is the son of a chalutzah (a woman taken in levirate marriage); this is the son of a divorcée. The sages say: He is not believed.