Slavery and revolution
Isabel and Curzon are both slaves living in America. The timing of the novel is inherently ironic, because while the Americans are actively disagreeing with the tyranny of the British government, both American and British tacitly casually accept the existential tyranny they commit by owning human beings as slaves. The context of the American Revolution is ironic because although Isabel and and Curzon are team America, they are literally slaves according to American law.
Moral corruption
Moral corruption is shown in all its ironic glory. Instead of feeling remorse or regret for her sociopathic tendencies, Lady Seymour considers herself basically morally good and conservative. She does not challenge the acceptability of her own moral values because she is morally corrupted by an assumption that she should be entitled to have things any way she wants them. Compare her entitled freedom with Curzon's imprisonment, and the truth about her moral corruption becomes apparent.
The importance of Isabel
When Isabel finds a tie to an active population of American rebels, she becomes a symbol for fate and irony. She ends up having vital information that the Americans need in order to succeed, and she even helps the Americans to prevent an assassination attempt on George Washington. This is ironic, given the American institution that keeps her as a slave. She is not just a person; she is a vital part of a serious movement, right at a critical moment of US history. This irony is highly symbolic, because often the assistance of Black Americans in American history is unfairly neglected.
Ruth's dramatic fate
Ruth slides from the realm of knowledge and order into the realm of silence and chaos. One day, Ruth is just gone. Isabel's owner tells her that she sold her sister to spare herself the discomfort that was caused whenever Ruth would have a seizure. Instead of empathizing with Ruth's medical concerns, she counts her as broken property and gets rid of her. In the end, Isabel learns that Ruth has not been sold. She is still in Lady Seymour's sociopathic possession, and Lady Seymour threatens to torture and murder the slave to spite Isabel. This leaves Ruth's fate within the confines of dramatic irony.
Freedom and fate
There is an abstract irony present in the novel which contrasts the freedom of human will with the fateful confines of circumstance. A person's free will seems infinite in some circumstances, like Lady Seymour's entitled privilege, and in other circumstances, fate seems to bind a person to intense suffering, like with Isabel, Ruth, and Curzon who experience the horror of being universally treated as inferior and dehumanized. They live incredibly difficult lives, but they are also awarded honor by the novel, and their importance clearly speaks for itself.
