Issac was the Only begotten Son of Abraham through Sarah. The children of Hagar and Keturah were not married to Abraham under the New And Everlasting Covenant of Marriage, so their children were not included in the Covenant. This is why Abraham sent them away with gifts, so that Abraham’s children through Sarah would not inbreed with Abraham’s children who were not of the Covenant. However it was different in the case of Jacob’s two wives and two concubines. They were all of the Covenant. Sarah was Abraham’s half sister and she was of the Covenant race, but probably Hagar and Keturah were not — This however does not mean that Hagar was Black. The children of Hagar and Keturah would not be brought under the Covenant until after the Times of the Gentiles was to be come in. On this count the marriage of Sarah to Abraham was a lot like the marriage of of Adam and Eve. Adam had other wives, but this world was reserved for Eve’s children, just as Sarah was to be the one and only Mother of the Covenant Race.
Reference | Hebrews 11:17-19