Let Me Explain
The writing style of The Rules of Survival is epistolary because it's told in the form of a long, confessional letter to Matthew's youngest sister, Emmy. According to Matthew, Emmy may not remember many details from their former life, so he's putting it down on paper so that she can read about their past when she's older:
I need to try and turn the experience into something valuable for you, and for myself—not just something to be pushed away and forgotten. (P.4)
Matthew goes into deep detail so that Emmy can learn about everything that happened. This is a long, in-depth explanation of their family's history… and how it's affected them all.