Does Calvin have any brothers or sisters?
No, Calvin is and only child. In one comic, Calvin is seen waking up at 5:00 AM on a Saturday bouncing off the walls and stairs and eating cereal with a 1:1 ratio of sugar to cereal. Calvin later comments that his habits are producing the desired effect; he has no brothers or sisters so far. So, in short, Calvin doesn’t want brothers or sisters anyway, he just wants Hobbes.
