Written as a sonnet, "Sisters" is a funny, half-serious portrait of rivalry between two siblings who look alike but could hardly be more different. The speaker is the older sister, yet the younger one is taller and more famous, and that stings. The two don't see eye to eye, and when one stumbles, the other laughs.
The speaker jokes that her sister must be a Martian, borrowing the idea of a "War of the Worlds" to make the family feud feel like an alien invasion. Beneath the humor is real hurt: she says her sister is cold and frustrates her almost to tears. But she is honest enough to admit she isn't innocent. She spills secrets and shifts blame, though she insists her sister is "worse times ten."
The mood is playful and teasing, then softens in the final couplet. Martian or not, this is still her sister, and their love may be the one thing they can count on.