Marital fidelity and normal life
The sisters have a few important differences between them. Firstly, Violet experiences premonitions more openly and readily than her sister, and she talks to her guardian spirit, simply named Guardian. Kate has long abandoned her own psychic powers.
On the other hand, Kate has a regular family life with a husband and two small kids. Violet doesn't have a husband and if things go well with Stephanie, she might not ever have a husband. In other words Violet is more open-minded and creative, and her sister wants a straightforward, meaningful life with a family. We see both sides of this coin throughout the book.
Sexual ethics
The question of sexuality is raised early in the book and remains a theme in the novel through to its end. For instance Violet's sister is not happy when she learns that Violet's new boyfriend isn't a boy at all; Violet has begun dating women, and she tells her sister that she is bi-sexual and has been. Her sister foolishly hopes it's just a phase, but by the end of the novel, it's Kate, not Violet, who has entered dubious waters in terms of sexual ethics. Kate ends up having an affair that will catastrophically effect the lives of her children, whereas Violet's attraction to women doesn't hurt anyone at all.
Premonitions, prophecy, and disaster
Since Kate doesn't like premonitions or prophecy, Violet's experience of those things seems even more credible, especially when Kate herself begins feeling the same premonitions as her twin sister. They know something important will happen, but they don't know what. Violet believes it will be an earthquake, but ultimately, the day sees not an earthquake, but a disaster of another kind: Kate sleeps with Hank while her husband is away with Hank's wife.
