Deuteronomy 21:16

16 then it shall be that whensoever he shall divide by inheritance his goods to his sons, he shall not be able to give the right of the first-born to the son of the loved one, having overlooked the son of the hated, which is the first-born.

Deuteronomy 21:16 Meaning and Commentary

Deuteronomy 21:16

Then it shall be, when he maketh his sons to inherit that
which he hath
By a will in writing, or byword of mouth, or by a deed of gift, actually bestowing his goods upon them, and dividing among them what he is for the present possessed of; see ( Luke 15:12 ) ,

that he may not make the son of the beloved firstborn before the son
of the hated, [which is] indeed the firstborn;
that is, when such is the case, that the son of his wife he has the least value for is really his firstborn, he may not, through favour and affection to the wife he loves better, prefer her son, and declare him to be the firstborn, by devising to him or bestowing on him the double portion of his goods; for so to do would not be right, or agreeably to the will and law of God; for though previous to this law the birthright was given to Joseph, the eldest son of Rachel, the most beloved wife of Jacob, before Reuben who was the son of Leah, less beloved by him, and was in fact his firstborn; yet this was owing to the sin of Reuben, and by the appointment of God; see ( Genesis 49:3 Genesis 49:4 ) ( 1 Chronicles 5:2 1 Chronicles 5:3 ) .

Deuteronomy 21:16 In-Context

14 And it shall be if thou do not delight in her, thou shalt send her out free; and she shall not by any means be sold for money, thou shalt not treat her contemptuously, because thou hast humbled her.
15 And if a man have two wives, the one loved and the other hated, and both the loved and the hated should have born him , and the son of the hated should be first-born;
16 then it shall be that whensoever he shall divide by inheritance his goods to his sons, he shall not be able to give the right of the first-born to the son of the loved one, having overlooked the son of the hated, which is the first-born.
17 But he shall acknowledge the first-born of the hated one to give to him double of all things which shall be found by him, because he is the first of his children, and to him belongs the birthright.
18 And if any man has a disobedient and contentious son, who hearkens not to the voice of his father and the voice of his mother, and they should correct him, and he should not hearken to them;

The Brenton translation of the Septuagint is in the public domain.