The ironic father
Caroline was a child when her (maybe) father contacted her privately and explained to her that her family tricked her, and that they weren't really her family, and that he was her real family. By lying to an impressionable child, he gained her company. This makes him ironic because although to Caroline, he is just her father, the reader knows that he committed a felony by abducting a young, helpless girl. This is an instance of situational irony and dramatic irony.
The ironic wilderness
When the social worker begins testing Caroline's education, she finds that not only is she educated, she has advanced academic abilities. This means that Caroline's father is a good teacher, and that Caroline has been taken care of in some important ways, so she lets them reunite, but she makes them leave their wilderness abode to live on a farm. At the farm, the father loses his mind. It seems that for Caroline and her father, society is difficult to stomach. Ironically, they were happiest when they had no modern amenities, living in caves.
The ironic twist
The plot twist for this novel shows that the narrator might not be trustworthy, because Caroline seems morally good, but at the end of the novel, she begins searching for a girl to kidnap. Caroline has become a paranoid abductress, like her "father." The journey from being the victim to being the villain is an ironic journey.
The irony of "normal life"
When Caroline gets a degree and a job, she discovers that she is unfulfilled by her role in society. The irony is that everyone feels that way sometimes, often for a very long time. Instead of patiently searching for her destiny like the rest of us, Caroline regresses back into her dysfunctional life and the consequence is that she wants to do an evil thing (kidnapping a new victim of her own).
The irony of Sisters
Sisters, Oregon is the city where Caroline loses her "father" and chooses to return to her—you guessed it—sister! The irony is that the city is called Sisters, which means that when Caroline gets to Boise and discovers that her old life isn't calling her back, she leaves her real, literal sister to go to Sisters, Oregon.
