Was the Genesis account of Ishmael and Isaac accidental, or did God set it up on purpose?

Les teaches that this whole scenario, including Hagar’s conception of Ishmael, his birth, her taunting of Sarai, and her being sent into the desert, was not random. The Lord sent Hagar back because 18 years later she would go out into that same desert, showing God always has the big picture in mind. Les explains that God can leap thousands of years as though it is a day. Every jot and tittle was programmed by God so Paul could use it as a tremendous lesson. None of this comes by accident. Later, Isaac is five years old and weaned, Ishmael about eighteen, and the same attitude of rebellion and taunt appears when Sarah sees Ishmael mocking, teasing the little five-year-old.