The boy's terrible decisions are a big part of the story. He was forced into a terrible situation and had to make decisions that he simply wasn't able to make. He should have stayed with his aunt, and she shouldn't have let him take off on his own. His stubborn childish behavior almost directly caused the death of his little sister and he didn't realize what he was doing to her until it was too late.
You're supposed to feel angry at the boy, but you also have to remind yourself that he was only 10 years old...I think. I can't remember. Kids generally aren't able to make long term life-and-death decisions like that.