Hastings' Dictionary of the Bible (1898)
INHER'ITANCE. T. we have no record of wills. The property-holder made a disposition of his property during his lifetime. There do not seem to have been very definite laws stipulating the exact proportion to be given to each heir. The sons had priority of right, and, in case there were no sons, the daughters became heirs. Num 27:8. As between the children of concubines and the children of legal wives, the latter seem to have received the whole inheritance, Gen 21:10; Gen 24:36, while gifts were bestowed upon the former. Gen 25:6.
However, while these principles were acted upon by Abraham, we dare not make the sweeping assertion that they were of universal application. Jacob blessed both sons of his concubines and sons of his legal wives. Gen 49:1 ff. According to Deut 21:15-17, the first-born son received a double portion. Believers have for their inheritance salvation, Heb 1:14, and the kingdom of heaven. Jas 2:5. They are declared to be "joint-heirs" with Christ and heirs of God because of their sonship. Rom 8:17.