Now Sarai Abram's wife bare him no children: and she had an handmaid, an Egyptian, whose name was Hagar. And Sarai said unto Abram, Behold now, the LORD hath restrained me from bearing: I pray thee, go in unto my maid; it may be that I may obtain children by her. And Abram hearkened to the voice of Sarai. Sarai ... took Hagar her maid the Egyptian ... and gave her to her husband Abram to be his wife. And he went in unto Hagar, and she conceived. Genesis 16:1-4Soon Sarah became jealous of Hagar and treated her so badly that Hagar ran away. Then an angel appeared and told Hagar to return to her mistress. So she did, and amid this messed up family life situation Ishmael was born.
When Sarai dealt hardly with her, she fled from her face. And the angel of the LORD found her ... And he said ... Return to thy mistress, and submit thyself under her hands ... Behold, thou art with child and shalt bear a son, and shalt call his name Ishmael. 16:6-11Fourteen years later, God helped Sarah get pregnant, and Isaac was born. (I’ll skip Bible quotes here.) Then after Isaac was weaned, Sarah saw the 16-year-old Ishmael “mocking” his two-year-old brother.
The child [Isaac] grew, and was weaned ... And Sarah saw the son of Hagar the Egyptian, which she had born unto Abraham, mocking. 21:8-9Some have suggested that Ishmael’s “mocking” of his younger brother Isaac was of a sexual nature, noting that the same word (metzahek) is used to describe the behaviour in this verse is used in Genesis 26:8 to describe Isaac's fondling of Rebekah.
Abimelech king of the Philistines looked out at a window, and saw, and, behold, Isaac was sporting [metzahek] with Rebekah his wife. 26:8If so, that might explain Sarah’s hostility toward Ishmael, since in the very next verse she tells Abraham to abandon Ishmael and his mother Hagar. (Although Sarah was hostile to Ishmael while he was still in Hagar’s womb.)
Wherefore she said unto Abraham, Cast out this bondwoman and her son. 21:10God, by the way, agreed with Sarah, telling Abraham to do what she said.
And God said unto Abraham ... hearken unto her voice. 21:12Maybe God saw what Sarah saw.