First, let's look at the capitalization… Oh wait—there isn't any. hush is lowercase, as if even the title is whispering.
There are lots of issues around silence versus speech in this book. When Daddy chooses to break the Blue Wall of Silence that says cops don't testify against other cops, he sets in motion a chain of events that lead to his family's relocation with witness protection. Toswiah/Evie talks a lot about how she can't tell the truth because her family is in hiding; she has to literally "hush" about her life before. So without being specific itself, the title clues us into the fact that we're entering a world of secrets in this book.