8 And the child grew, and was weaned: and Abraham made a great feast the same day that Isaac was weaned. 9 And Sarah saw the son of Hagar the Egyptian, which she had born unto Abraham, mocking. 10 Wherefore she said unto Abraham, Cast out this bondwoman and her son: for the son of this bondwoman shall not be heir with my son, even with Isaac. 11 And the thing was very grievous in Abraham’s sight because of his son.
We learn a new detail in verse 9, that Hagar was an Egyptian. Presumably she was recruited during their visit to Egypt at the time of the famine. There is an interesting foreshadowing in this moment that Ishmael, a son of Egypt, afflicts Isaac, a son of Abraham. This same pattern will play out on a much larger scale when the entire nation of Israel is made slaves to the Egyptians, awaiting deliverance at the hand of Moses.
Now comes a difficult situation for Abraham. His two sons are divided against each other, and Sarah is filled with indignation for the offense to her son. Naturally Abraham cleaves to both of his sons, they are each his own blood. But Sarah is only tied to Isaac and her concern is strictly for him.
Now we do not know exactly what she saw in Ishmael’s mockery. “Mockery” is a very wide term. It might mean anything from harmless teasing to hateful tormenting. It is possible that Sarah something that made her fear a terrible violence when she and Abraham, Isaac’s already-old parents, went to the grave and were no longer around to protect him.