Charlotte Light And Dark was a book written about Brenda when she was a child by her psychiatrist. Because of HIPPA laws and the fact that she was a minor, I'm assuming her parents (also psychriatists) signed away the rights to her story. Brenda would mess with her doctors and read up on psychiatric disorders and mimic the symptoms.
Brenda has borderline personality disorder (Google itits very interesting)

You may be thinking of Lisa, Bright and Dark - which seems to be very similar to the book written about Brenda was.
https://www.amazon.com/Lisa-Bright-Dark-John-Neufeld/dp/0451166
The children's books looked to be a take on the Lemony Snicket series, just changed to a brother and sister team of Nathaniel and Isabel. They were pretty dark (as Roald Dalh used to write) and Billy seemed to see a connection between him and Brenda.

I meant Brenda not Claire. I was watching a scene with th Claire when typing