sams crossing the thresh hold was when he made it out of the sack the adults stuck him in and escaped from the stadium where most of the adults had sort of moved in.
tests and helpers was when he was taken by adults who werent zombies and took refuge under ground in the train station. When the adults stepped into the light they became zombies
My book is the first in a seriese of several so the hero has not found his sister but him and the kid have found refuge with a group of kids that had food and shelter.