Symbolism, Imagery, Allegory
Gemma's obsession with her favorite fairy tale, Briar Rose, puts Justin Bieber fans to shame. The reason that she's so into it is that the fairy tale is the coded story of her life—a sort of fictional biography.
Where everyone else in her family thinks that Briar Rose is just a silly bedtime story, Becca starts to recognize that there's meaning behind it, even before Gemma's death. "It's not that I believe it," she tells her sisters. "Or even that she does. It's like the story…is like a metaphor…." (2.57)
In fact, Gemma's story contains a lot of different metaphors, so technically it's an allegory. That's English-teacher talk for a story with hidden symbols and meanings.
Let's review the details of the story, which Becca matches up to events in Gemma's real life as she investigates the past.
Gemma says she was a princess in a cursed kingdom. The curse was Nazism—which a Holocaust survivor in Fort Oswego helpfully points out to Becca when he says, "The Nazis were the curse." (12.135)
Gemma's story also says that everyone in the kingdom went to sleep. Yep, you guessed it, sleep stands for death, and Gemma's kingdom was an extermination camp.
Finally, Gemma says that she—and only she—woke up when the prince kissed her. As Becca learns in Poland, the prince is Josef P, and his "kiss" was mouth-to-mouth resuscitation. Hey, we'll take the gift of life over romance any day.