Clara isn't Morticia Addams, but she is a little morbid. Clara isn't weird, but she is a little quirky. Most of all Clara isn't a big sister, but she was going to be one.
Now 16, all of Clara's ups and downs, worries, thoughts, and memories are being written out, and addressed to Sam. Sam, the baby who didn't make it, Sam the kid who didn't get to experience all of the wonderful things Clara has. Clara , drawn to the thought that somewhere, somehow that baby is growing and missing her to. So Clara begins to write letters, hoping that Sam will get wind of them.