When Julian’s mother does not reply to Auggie’s birthday invitation, Auggie’s mum says, ‘I guess the apple doesn’t fall far from the tree.’ What does this saying mean? (p.67)

Julian is the story's main antagonist: a kid at school who is consistently rude to Auggie, and tries to turn the entire school against Jack when Jack decides to keep being Auggie's friend. Auggie's mother thinks the reason Julian is so rude might be because his mother is rude.