Character Clues
Character Analysis
Actions, Family Life, Thoughts and Opinions
We really only get to know the character's through Alexander's eyes, so it's worth noting that their actions should be taken with a grain of salt. That said, it's pretty clear that Anthony and Nick aren't really supportive of their little brother, and we get a good idea of Mom's parenting style through her apparent standoffishness. Even Alexander's own accounts of his actions subtly reveal clumsiness in his dad's office, messiness in his own room, and a bit of mischievousness in school. And his thoughts and opinions about all of these events reveal just how childish he really is.
Of course, a lot of our understanding of the characters in Alexander's family especially comes through our own concepts of what family dynamics are supposed to be. Big brothers aren't supposed to be nice, so Anthony and Nick are totally real characters.