Answer:
Ellie and the narrator are cousins.
Step-by-step explanation:
The short story Jeremiah´s Song by Walter Dean Myers, starts by clarifying this relationship between the 9-yr old narrator and Ellie: "I knowed my cousin Ellie was gonna be mad when Macon Smith come around to the house."
Throughout the story. Ellie seems to have a distant relashionship with him, as he describes how she has changed since she went to college. She does not seem to have interest in spending time with him and do the things they used to do, like when he asks her to sleep with him, and she turns him down.
Ellie has grown and her interests are different now.