“Now Sarah saw the son of Hagar the Egyptian, whom she had borne to Abraham, mocking. Therefore she said to Abraham, “Drive out this maid and her son, for the son of this maid shall not be an heir with my son Isaac.”” Gen. 21:9-10
Sarah demands Abraham send Hagar and Ishmael away after seeing something she dislikes in Ishmael’s interaction with Isaac. The NASB translation uses the word “mocking”, but we do not know what happened for certain. There is wordplay in the original Hebrew which seems to indicate that Ishmael was ‘playing’ at being Isaac – that is, pretending to be Isaac in some way.
Jewish historian Josephus understood Sarah’s concern as being that since Ishmael was 14 years older than Isaac, if jealousy of Isaac were to arise within Ishmael, who would protect Isaac from him when Abraham passed away? (Antiquities, Book 1, 12:3) We can only wonder if secretly Sarah was concerned about a repeat story of Cain and Abel, thus preventing Isaac from receiving the inheritance. After all, the general practice was to give the inheritance to the older son, and she could not let that happen.
Notice in verse 10 that Sarah does not even utter the names of Hagar and Ishmael, but rather refers to them as ‘the slave-woman and her son’. This may be done out of anger, or may indicate there has already been an emotional detachment on her part. As we see in verse 11, Abraham is grieved over ‘his son’, referring to Ishmael, so he does not appear to share these feelings with Sarah.