The First Born: Privileges of not to be Alienated by Parents Through Caprice
Source: TTT
Deuteronomy 21:15, 16
If a man has two wives, one beloved and the other unloved, and both bear him sons, but the unloved wife has the firstborn son, / when that man assigns his inheritance to his sons he must not appoint the son of the beloved wife as the firstborn over the son of the unloved wife.