16 when he wills his property to his sons, he must not give the rights of the firstborn to the son of the wife he loves in preference to his actual firstborn, the son of the wife he does not love.

Read Deuteronomy 21:16 Using Other Translations

Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn:
then on the day when he assigns his possessions as an inheritance to his sons, he may not treat the son of the loved as the firstborn in preference to the son of the unloved, who is the firstborn,
When the man divides his inheritance, he may not give the larger inheritance to his younger son, the son of the wife he loves, as if he were the firstborn son.