The outcome of this incestuous act was two sons.
Moab and Ammon: Both were the patriarchs to the areas named after them. They were not part of the blessing of Abraham but they also were not cursed because of their mothers’ sin. Throughout history both areas had up and down relationships with Israel. Moabites were polytheists and encouraged Israel to partake in worshiping their gods and Ammonites forbid Israel from passing through their land during the Exodus. It is at this point Lot and his family’s story comes to a conclusion.
Genesis 21:1 - 11
Vs 1 - 4: “… The Lord did to Sarah as he had promised.” For decades God has promised He was sending a natural born son to Sarah to be the sole heir of the blessing of Abraham. Finally the promise is fulfilled. Ironically it is with little fanfare. After decades of faith testing, impatience and human interference God does the impossible. Sarah became pregnant and give birth to her first son at the age of 90. This is significant and is a major part of the story because it shows us two things (both of which we already know)…1) God always fulfills his promise. 2) He is a God of miracles. He can and does do the unthinkable and the impossible for his glory. Once the child is born Abraham names him Isaac (which means “He laughs”) and has him circumcised on the eighth day.
Vs 5 – 7: Abraham is a father of a newborn baby boy at the ripe ole age of 100. This is mentioned so we can see that what may have seemed impossible had actually become a possibility. The birth of Isaac was a divine thing. After the birth Sarah proclaims her excitement through laughter as she says, “God has made laughter for me;” If you recall Sarah’s laughter in chapter 18 was in cynicism, unbelief and hopelessness but now merely a year or so later her laughter is one of joy and jubilee. The promised son brings unspeakable joy to Sarah. She then declares that when people hear her story they will laugh with her because God has done the impossible by giving an old woman well past her child rearing age a child of promise. This story will ultimately bring universal pleasure and joy because of what God has done for her. They will hear this story and say, “Who would have thought that such an old couple could or would give birth to a child?”
Vs 8 – 11: Once the child was weaned (probably at the age of three) Abraham had a party to celebrate a momentous milestone in the child’s early life. According to Gordon Wenham this is noteworthy because, “In a society where infant mortality was high, to reach the age of two or three would be regarded as a significant achievement, so this part explains the magnitude of the celebrations. From now on Isaac looks relatively certain to be Abraham’s heir.”
Unfortunately Sarah’s joy was cut short as she hears Ishmael (notice the account does not name him; instead the text refers to him as “the son of Hagar the Egyptian.”) laughing. Some translations say he was “mocking” and others say “scoffing”, “Poking fun” or “laughing”. The word used is the root of the word Isaac but may imply more of jesting, or making a sport of. It is not clear what was actually going on exactly. It is believed he was either mocking or making fun of Isaac or he may have been playing or being playful with Isaac as though they were on equal footing (equal brothers). If he was mocking then we see why Sarah gets mad. However if he was playing or being playful with Isaac it would seem a little harder to understand her reaction until we realize she reacted this way because he was acting like a big brother who would one day inherit the blessing of the father. Her maternal instincts kicked in here for the reason why she wanted Ishmael and Hagar gone. If they weren’t around then there would be no question as to who the rightful heir is.
Sarah demands that Abraham drive out (the same word used for divorce) Hagar and send her and Ishmael off. This displeases Abraham greatly and rightly so. I believe here we get a glimpse of the paternal instincts of Abraham. Ishmael WAS his son regardless of who the mother was. Abraham had strong affections for his son, like any caring father would. So when his wife demands he send them both away this must have broken his heart. Unfortunately in this time of great rejoicing (celebration of weaning) Abraham is quickly met with a time of great despair. This is not uncommon even for us today; to have a mountaintop experience only to find ones self in the valley of despair shortly thereafter.
 Wenham, G. J. (1998). Vol. 2: Genesis 16 - 50. Word Biblical Commentary (81). Dallas: Word, Incorporated.