John Gill Commentary Deuteronomy 21:15

"If a man have two wives, the one beloved, and the other hated, and they have borne him children, both the beloved and the hated; and if the first-born son be hers that was hated;" — Deuteronomy 21:15 (ASV)

If a man have two wives
Which is supposed, but not approved of, though permitted because of the hardness of men's hearts; for it was not so from the beginning, when only one man and one woman were created, and joined together in marriage; but as it was connived at, and become customary, a law is made to prevent confusion, and preserve order in families:

one beloved and another hated ;
or less loved, yet continued his wife, and not divorced. Aben Ezra observes, this follows upon the former, because it is there said, that though first he had a desire to her (the captive beautiful woman), yet afterwards had no delight in her:

and they have borne him children both, the beloved and the hated ;
as Rachel and Leah did to Jacob, who were, the one very much beloved by him, and the other less:

and if the firstborn son be hers that was hated ;
or not so much beloved as the other, as was the case in the above instance.