Now Sarah saw the son of Hagar the Egyptian, whom she had borne to Abraham, mocking Isaac. Therefore she said to Abraham, “Drive out this slave woman and her son, for the son of this slave woman shall not be an heir with my son Isaac!”
Genesis 21:9-10 (NASB)
There is a story in the Midrash that paints a sweeping narrative that creates context for these two verses. It starts back in the previous chapter, when Sarah is taken by Abimelech.
So Abraham prayed to God; and God healed Abimelech, his wife, and his female servants. Then they bore children; for the Lord had closed up all the wombs of the house of Abimelech because of Sarah, Abraham’s wife.
Genesis 20:17-18 (NKJV)
Remember: the healing that happens in that chapter is related to childbirth.
Based on this childbirth-related healing, and then the birth of Isaac in chapter 21, the rabbis suggest that rumors began to spread. Perhaps this son of Sarah is no son of Abraham, but instead is a son of Abimelech. The timing is right, and clearly, Abimelech’s wife and “female servants” are all pregnant.
While we, the readers, are told that God prevented Abimelech from touching Sarah, who knows if the people in the story, to include Abraham’s whole entourage, believe it?
And most importantly, does Hagar believe it? She has witnessed Abraham and Sarah’s frequent deception first-hand.
So the rabbis say this: “the prattle of children reflects what they picked up from their father or what they picked up from their mother.” If Ishmael is “mocking” Isaac (and not abusing, per another interpretation of the story), perhaps the mockery is this: “Everyone knows Abraham is my father. Who is your father?”
And Sarah, hearing this, would know that these are Hagar’s words, and that Hagar would be making a clear statement about inheritance: Only the son of Abraham should receive any portion of Abraham’s blessing and wealth.
Perhaps this is why Sarah declares, “the son of this slave woman shall not be an heir with my son Isaac!”