The novel suggests a dichotomy between two points of view, the view of either sister. Whereas Monica is primarily concerned with her performance in her external world, defining success to be a certain feeling of dominance and power, Claire finds power in private, by exploring her body after learning about masturbation at a conference. The implication makes a subtle but powerful point: Without empowering female sexuality, the girls struggle to find an outlet for their repressed identities.
That doesn't mean that masturbation is some kind of perfect religious trade where a person can just delight themselves into ecstasy (it isn't). What it means instead is that the girls have been made to feel shame about their sexuality, and that has placed certain borders in their personalities. Monica struggles with listening to Claire, because she is more dominant in her repressed identity, and Claire finds herself in quiet, kinky roles of passiveness.
The novel's dichotomy is also suggesting something else: balance and diversity. Just because these sisters are different, they are still loving sisters—perhaps they often perplex each other, but mostly, they are friendly. In the end, Claire feels she must confront Monica, and they both apologize to each other. Perhaps this sisterly peace is a symbol of the novelist's desire for women to encourage each other in their journey toward manifesting their full potential.