"So he went in to Hagar, and she conceived. And when she saw that she had conceived, her mistress became despised in her eyes" (Genesis 16:4). The relationship between Sarah and Hagar quickly deteriorated and Hagar fled.... You are now with child and you will have a son. You shall name him Ishmael ['God hears'], for the LORD has heard of your misery. He will be a wild ass of a man; his hand will be against everyone and everyone's hand against him, and he will live in hostility toward all his brothers" (verses 10-12, New International Version).
Fourteen years after the birth of Ishmael, God blessed Abraham with another son, this time by his wife Sarah. He told them to name their son Isaac (meaning "laughter" for the incredulous reaction they had when told they would have a son at their advanced age as well as the joy that he would later bring to his parents, Genesis 17:17, 19; 18:10-15; 21:5-6). Isaac in turn fathered Jacob, also named Israel, the father of the Israelites. Ishmael's and Isaac's descendants are therefore cousins.
"So the child [Isaac] grew and was weaned. And Abraham made a great feast on the same day that Isaac was weaned. And Sarah saw the son of Hagar the Egyptian, whom she had borne to Abraham, scoffing. Therefore she said to Abraham, 'Cast out this bondwoman and her son; for the son of this bondwoman shall not be heir with my son, namely with Isaac'" (Genesis 21:8-10).
"Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of ONE, And to thy seed, which is Christ. (Gal 3:16KJV)