On page 56, what does Scout mean when she says, "It was then, I suppose, that Jem and I first begin to part company."

Jem takes time to process the event when his pants were mended before he tells his sister what happened. This shows Jem as putting some distance between the two of them. It shows a maturity that makes Jem independent from his little sister, “It was then, I suppose, that Jem and I first begin to part company.”