Once, someone rang me from a hotel room, insisting the party Bella and her friends had cajoled her into having had got out of hand. “Are you Bella’s sister?” she asked, and when I said I was, she ordered me to come and help her clean it up.

In literature too, birth order is a common concern. When the youngest sister in Pride and Prejudice elopes before the others are even engaged, the whole structure of the family is threatened. The literary world has equally long been fascinated by the rivalry between two of its leading novelists, the late A. S. Byatt and her sister, Margaret Drabble. Recently I heard Drabble—the younger—explain in an interview how her sister never forgave her for having the temerity to publish a novel before she’d had a chance to do so, not when she knew her sister’s ambition had always been to write. Even when Byatt went on to win the Booker Prize, the bitterness between them never entirely dissolved.

Fortuitously, my debut novel, Hideous Kinky—based on the years Bella and I spent traveling in North Africa with our mother when we were very small—was published the same year that Bella launched her womenswear collection, winning young designer of the year, while my book was optioned for a film. We were bonded, all over again, not just by our challenging and peripatetic childhood—the need to keep each other safe—but by the adventure of our self-made lives.

In the decades since, we have watched each other closely, comparing our successes and our failures in both work and love. Striding ahead, falling behind. Every new pitfall is an opportunity for discussion. Hours and hours of conversation, every last detail of our lives examined.

“I wish I had a sister,” my own daughter has sometimes complained, sandwiched as she is between two boys, and I have wondered who she’d be if I had provided one. There have been other times too, when, shocked by Bella’s forthrightness, my daughter has looked at me, appalled. “Or maybe,” she’s conceded, “it’s good to be the only girl.”

“I don’t know how to… hate… you,” I hum to myself. No one can know, if they’re not embroiled, the heady pleasure of feeling yourself so entwined. Just last week I was walking across Hampstead Heath when a figure called out through the bushes: “Are you Bella’s sister?”