In the book, there was a slave girl who works for a loyalist family during the revolutin. She was separated from her sister, who had recurring seizures, since the master's wife thought she was possessed. In the ending, her and her guy friend ran away

Also the family were loyalist spies, and the girl helped the Patriots exploit their secrets. The Head Mistress was exceptionally mean, and had her branded "I" for insolance

Think maybe its this:
Chains (http://www.thrivingfamily.com/Family/Media/book-reviews/c/chains.aspx)