Well, it is interesting, if I remember correctly, that Isaac, 2nd born of Abraham and Ishmael, first born (founder of Islam so to speak) are brothers. Ishmael's mother is a slave girl that Abraham with God's permission slept with to have a son since his wife Sarah was unable to have a child. Then she somehow got pregnant with Isaac. Ishmael instead of inheriting his rightful destiny as first born instead watched it pass to his younger 1/2 brother Isaac. Ishmael, left and as we know it today you have Islam and Judaism. In fact one might say that this is why the Muslims hate the Israelites so much and the actual root of the violence we have today.
Actually (according to the bible), Abraham had Sarah's permission to sleep with Hagar (Sarah's handmaid), not God's permission. While I'm sure there would be a lot of arguments supported by differing texts, the bible supports that God said that it would be through Isaac that his promise to Abraham would be fulfilled. But, that god would also make Ishmael a "great nation" I believe it was.
Abraham sent Hagar and Ishmael out to wander the wilderness on Sarah's insistence as well. He was apparently pussy whipped.
Who the fuck knows what really happened though.