JOEL - Even though Abraham had one other son(Ishmael by Hagar) before Isaac was born, it was Isaac that God established His covenant with(Gen. 17: 21), and as such could be called Abraham's only "begotton son", being a similitude of the Father's only begotton son Jesus Christ.(Jacob 4:5.)
Since Abraham was 100 years old and Sarah was 90 (Gen. 17:17), neither expected to become parents. Before the announcement of Isaac's birth was made, we must assume Abraham and Sarah expected the promise would be fulfilled through Ishmael, born to Hagar, Sarah's handmaid. (Gen. 16.)
However, the contempt Hagar unexpectedly demonstrated toward Sarah after conceiving, set in motion the unfortunate events that resulted in Hagar's eventual exile and the fulfillment of the covenant through Sarah and her son Isaac.

The Lord seems to have set the exact time when Isaac would be born (Gen. 17:21; 18:14; 21:2), and revealed the name by which he would be called in mortality. (Gen. 17:19; 21:3.) Thus Isaac became the first (and only) son of the first wife, making it culturally and traditionally appropriate for him to receive the promise made to his father.

Just as the Jews of today regard themselves as decendants of Abraham through Isaac The Arabs(mostly Moslems) of today regard themselves as descendants of Abraham through Ishmael and place Ishmael at least equal with Isaac. They claim that it was not Isaac who became the true heir of the covenant, but it was Ishmael. So there is always this fierce rivalry between the two peoples.
The Arabs believe Ishmael was one of Allah's prophets as mentioned in the Koran:

"And make mention in the Scripture of Ishmael. Lo! he was the keeper of his promise, and he was a messenger of Allah, a Prophet. He enjoined upon his people worship and almsgiving, and was acceptable in the sight of his Lord." (Koran, Surah XIX:54-55.)

I am no prophet so I don't know much about the future of Ishmael's posterity. The Lord did however give a great promise regarding Ishmael and his descendants:

"And Abraham said unto God, O that Ishmael might live before thee.
And as for Ishmael, I have heard thee: Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation. (Gen. 17:18, 20)

A promise which has been fulfilled to some degree. Even though the Lord made covenants with Isaac, any child of Ishmael may have as many blessings in the Church or in eternity as any child of Isaac if he will but serve Almighty God and keep his commandments. Also, any child of Isaac, regardless of heritage, may lose those blessings if he fails to keep the commandments of the Lord.