In From the Mixed-Up Files of Mrs. Basil E. Frankweiler, Claudia and Jamie stick together through thick and thin, and it's not because they love being around each other all the time. Nope. Even though Claudia can be insufferably snobby, and Jamie can be a bit of a cheapskate, they stay together because they're family.
Even though they already have a family, they seems to be a search for something more. After all, they don't really like Kevin the brat and their parents seem pretty focused on him. In the end, they find Mrs. Frankweiler and adopt her as a surrogate grandmother. Since they don't have their own grandma and she doesn't have her own grandkids, it turns out to be a happy (albeit strange) arrangement.
Moral of the story? Maybe family isn't about who you're stuck with. Maybe it's more about who you choose to be stuck with.
Even though Claudia and Jamie were always siblings, during the course of running away and staying at the museum, they really learn what being family means—working as a team even if they have scuffles and arguments sometimes.
In the end, finding Mrs. Frankweiler and getting to know her is just as important as learning the secret of the statue. Claudia and Jamie gain a new "family" member along with their knowledge of Angel's origins.