The cover of Richard Peck's novel A Long Way From Chicago shows a boy with a man in plane and an older woman holding the hand of a young girl under the flying plane. One can easily assume that the two males in the plane are Joey (the narrator and protagonist) and a pilot. Therefore, it could also be assumed that the two girls seen on the cover are Grandma Dowdel and Joey's sister, Mary Alice.
Given that Richard Peck does not disclose why he choose the image for the cover, one can only assume that it was chosen based upon the fact that the story depicts Joey flying in a plane. Joey flew in the plane at the county fair during one of the eight sumers he and Mary Alice spent with their grandmother in a small Illinois town.