Family and inclusion
This family is inclusive in ways, but often that inclusivity stops a little shy of Sophie. Because her adoptive family doesn't appreciate her struggle, because they ignore the truth about her past, she feels excluded. Because she is adopted, there is the question about the validity of her presence in the family. They act like she's family, but do they include her the same way they naturally include each other? For Sophie, the answer lies in journeying to Bompie, who is family to her in a different sense. He will validate her perspective, and that should help the problem, she feels.
Distrust and "otherness"
They can deny it if they want, but at the end of the day, the family shares something with themselves that they don't share with Sophie: Trust. They act like her stories are made up, simply because they are fantastic, but it is precisely their fantastic nature that makes Sophie so forceful about them. It is important that they understand that her point of view has unique challenges, but they don't trust her. This makes them treat her like she's "other" than them, as if she doesn't fit in, but that's exactly what she is trying to explain to them—why she doesn't quite fit in with their sense of normal.
Knowing the truth about one's past
In this book, Sophie is driven by an overwhelming emotional urge to validate her perspective. Her adoptive family doesn't acknowledge what is true about Sophie's past, which is a serious problem, because it means that Sophie will be left to fend for herself emotionally—they might mistreat her, not believing that she has suffered what she has. Sophie needs them to know the truth, but also, she herself needs to have the catharsis of being understood and believed. This helps her to admit the truth to herself.
