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.