I presume you’re referring to ol’ Ismael, Abraham’s bastard. For all we know, when he moved to the sands of Arabia, he met another Semitic tribe and married into them.
We do know what happened. Genesis 17:21 plainly says that Hagar, Ishmael’s mother, found him a wife from Egypt (Mizraim, who are Hamite) after they had settled in Paran. Genesis 25:13-15 lists the names of Ishmael’s twelve sons.