5 And Abraham gave all that he had to Isaac. 6 But Abraham gave gifts to the sons of the concubines which Abraham had (Genesis 25:5-6a)
Isaac was the son of promise. He was to be the continuation of the godly line, and as such, he was eligible for the inheritance. The Law of God nominates a double portion for the first born son of a man's progeny. However, in many biblical instances, it was other than the first born son who received the double portion. The reason is that God is much more concerned with the actually continuation of the presence of godliness in the earth, than he is concerned with legalistic continuation of a rule, even when the application of the rule will breed ungodliness.
The end point of all of God's Law is His glory and the extension of His Kingdom in time and on earth. His character is the measure of godliness, and his character is expressed in the plain reading of His Law. However, sinful men twist the reading of the Law, and apply it selectively, or blindly without consideration of the results.
The inheritance is a good example. In a godly family, where godly children have been raised, the normal situation would be to pass on a double portion of the inheritance to the eldest son, who in turn would have the responsibility of looking after the aged parents in their declining years, using half of the double portion to support the parents, and the other half of the double portion to provide for his children and work towards building them an inheritance.
However, we do not live in a perfect world, and sometimes the eldest son is a reprobate (as was Esau). In this instance, the double portion (the birthright) was given to a younger son (Jacob). God's Law was not followed to the letter, but the intention of the Law was fulfilled.
Abraham gave all that he had to Isaac, but only a gift to Ishamael. Isaac was the promised seed, and the godly heir, even though Ishmael was the first born.
As Christians we must be discerning in our allocation of the inheritance we gather throughout our years of labour in this world. We are to lay up for our children, and our children's children, however, we must be careful not to reward godlessness. Just because such and such a child is born to us does not mean that there is an inviolable Law that compels us to give to the wicked members of our family. NO! We should let our family know that godliness will be rewarded, and ungodliness will be disinherited. To give to the cause of reprobation is sin. While my sons are in my home they will respect my God and His Law-Word. Should they choose to move from my home and pursue other gods and other law-words, then they will be disinherited (and they have been told so). However, should they continue in the faith of their father, then at some time they shall be rewarded with the fruits of their father's labours to assist them with building their own godly family.