What is the main idea of Chains by Laurie Halse Anderson?

The main idea of Chains by Laurie Halse Anderson is the hardship of slavery before and during the American Revolution.  This main idea is furthered through the character of Isabel, a young slave, who lives with abusive owners (the Locktons) in New York City.  Isabel always seems to be teetering on the edge of slavery and freedom.  She is originally promised freedom by her previous owner, but when that owner dies, she is promptly sold.  It is from the Lockton's residence that Isabel learns the most about the fight between the Loyalists and the Patriots.  The main idea of slavery during this time period is furthered when Isabel's sister, Ruth, is sent away to Charleston.  Isabel vows to find Ruth.  The reader never finds out whether Isabel completes this task; however, the book ends with Isabel free and on her way to Charleston.