Unfortunately, readers are not given this information. Earlier, readers and Isabel assumed that Madam Lockton sold Ruth and that the likelihood of Isabel finding her is very unlikely. Then in chapter 43, Isabel bravely defies Madam Lockton's demand to hand over a note. Isabel throws the note into the fire, and Madam Lockton is so angered that she threatens to sell Isabel and Ruth. Isabel and readers are caught by surprise because we learn that Madam Lockton didn't sell Ruth.
"Couldn't find a buyer. Had to ship her down to Charleston. I shall tell the estate manager to get rid of her, toss her in the swamp."
It was a lie to intentionally upset Isabel. Madam Lockton couldn't find anybody that wanted to buy Ruth presumably because of her mental disorder. This information propels Isabel into escaping the Lockton household by forging her freedom papers and escaping New York. The book ends by giving readers the impression that Isabel will seek out and find Ruth. Book two in the series is focused on Curzon. He and Isabel did not go together to find Ruth, and readers will have to continue on to book three in the series for further information about what happened to Ruth after Madam Lockton sent her away.
This is something that the novel is not clear about. In Chapter 43, Madam reveals to Isabel that Ruth has not actually been sold, as she was led to believe, but that she has been living at the Locktons' estate in Charleston. Madam taunts Isabel by telling her that she will now punish Isabel for her disobedience and crimes by deliberately seeking out "the most cruel, the most heartless master" for her:
"Brat," Madam spat. "Couldn't find a buyer. Had to ship her down to Charleston. I shall tell the estate manager to get rid of her, toss her in the swamp.
Madam locks Isabel in the potato cellar and says she will sell them both the next morning. Isabel therefore determines to escape now and walk to Charleston to get her sister back. The rest of the novel concerns her escape and how she found a blank pass that she wrote her name in, and then her rescue of Curzon before they row together across the river to Jersey. However, no mention is made at the end of what happens next. Presumably the reader is meant to infer that Isabel and Curzon try and rescue Ruth, but the novel remains blank about her fate.