We know from the previous chapter(s) that Abram and his wife were promised Innumerable children from the Father. Given that their ages were getting up there in years Sarai had the idea that her Egyptian handmaid Hagar could become the mother of her children. After running the idea by Abram, Abram went along with the idea. This was not that out of the ordinary because of the law code of Hammurabi allows for such a situation although it does go against the Father's law.
As a result, Abram and Hagar had a child called Ishmael, which means "God Hears." Ishmael was Abram's oldest son and an angel tells Hagar he will be a wild man. Ishmael's hand would be against many and many will be against him as he will dwells in the presence of his brothers. The sin of the flesh between Abram and Hagar as it is called in Galatians 4:21-31 is only the end result of the underlying sin of not trusting the Father in delivering children for Abram and Sarai.
It's important to realize that Ishmael was the first born son of Abram, but did not receive the promise. The migrations of the descendants of Ismael are not well recorded in history. However, I have been told that some Arabs claim they are descended from Ismael and this could very well literally be the case.
There is one more thing I feel I should include in this post, the sins and iniquity of the fathers are passed down to their children according to the bible in Exodus 34:6-7 and in Deuteronomy 5:8-10. When any given father of a household sins, it degrades and opens the doors for their children to the third and fourth generation to sin also. The point I am trying to make is that there is a cascading effect when sin is committed both on yourself and children as it degrades the family structure. Below shows the sins of Abram as well as the third and fourth generation.
In this case this "sin of the flesh" has been documented to the third and fourth generation:
Abram Genesis 16:4
Abram's son Issac Genesis 25:21 Issac trusted the Father!
Abram's grandson (Jacob Israel) Genesis 30:3,9
Abram's great grandson Ruben Genesis 35:22
Praise the Lord for his word!