The Little Brother is a Figment of Imagination
In Radio Flyer, Two brothers are the victims of their widowed mother's violent drunkard husband who spares no rod with the youngest brother. Reverting to a world of make-believe, they imagine that their Radio Flyer wagon can fly and that in it they can escape their tormenting stepfather. At the end of the film, their makeshift plane, seemingly real this time, sends both of the children up into the sky and away from their abuse. This theory states that the little brother, Bobby, was a figment of imagination, dreamnt up by the older one, Mike, in order to cope with the pain that the father inflicted on him. Mike imagines Bobby getting abused whenever Mike, himself, gets beaten by his father. Its notable that no one besides Mike's family interacts with Bobby. Furthermore, the narrator's final lines ("Now do you understand what I mean about history being in the mind of the teller? [...]'Cause that's how I remember it.") lend further credence.