Ishmael was the son of Abraham and Hagar, Sarah’s handmaid (Genesis 16:15), and the Ishmaelites were his descendants.
Midian was the fourth of the six sons of Abraham and his wife, Keturah (Genesis 25:2), and the Midianites descended from him.
Ishmael was not the father of the Midianite nation, but the Ishmaelites and Midianites are so closely associated that it is hard to distinguish between them. When Joseph was sold by his brothers to the traders heading for Egypt, in Genesis 37:25-28, the traders are referred to as Ishmaelites and Midianites, interchangeably.
See Who were Jethro’s ancestors? for further discussion.