Actually, if we’re going to get all technical, after Sarah died Abraham had six more sons with Keturah. And quite probably some daughters.
The seed of Messiah was only passed to Isaac, and then to Jacob.
By faith Abraham, when he was tried, offered up Isaac: and he that had received the promises offered up his only begotten son, Of whom it was said, That in Isaac shall thy seed be called: Accounting that God was able to raise him up, even from the dead; from whence also he received him in a figure. By faith Isaac blessed Jacob and Esau concerning things to come.