Laurie Halse Anderson
Isabel is the main character, and the rest of the story takes place in New York while Isabel and Ruth are slaves there.
Isabel tells Miss Finch's brother about Mary's will, but he thinks she's making it up. And to make it all worse, Isabel's lawyer is in Boston at the moment and can't help them. So Isabel and Ruth can't be freed.
Isabel and Ruth get sold to the Locktons. Isabel starts working a double life as a patriot spy and a slave. Ruth has multiple seizures and Madam thinks she is possessed so she sells Ruth and makes sure Isabel can't fight back. Then the patriots get captured along with Curzon, and they get treated like slaves by the British. After Madam finds out Isabel is working for the patriots, she threatens to drown Ruth. So Isabel breaks out of the house, breaks Curzon out, who is passed out, and sails away from New York.
The rest of the plot takes place in the other two books of the trilogy.