She had always been there, and I could not have loved Ma any more if she had given birth to me.

During the first three years of my childhood, Marina and I had lived alone together in our magical castle on the shores of Lake Geneva as Pa Salt travelled the seven seas to conduct his business. And then, one by one, my sisters began to arrive.

Usually, Pa would bring me a present when he returned home. I’d hear the motor launch arriving, run across the sweeping lawns and through the trees to the jetty to greet him. Like any child, I’d want to see what he had hidden inside his magical pockets to delight me. On one particular occasion, however, after he’d presented me with an exquisitely carved wooden reindeer, which he assured me came from St Nicholas’s workshop at the North Pole itself, a uniformed woman had stepped out from behind him, and in her arms was a bundle wrapped in a shawl. And the bundle was moving.

‘This time, Maia, I’ve brought you back the most special gift. You have a new sister.’ He’d smiled at me as he lifted me into his arms. ‘Now you’ll no longer be lonely when I have to go away.’

After that, life had changed. The maternity nurse that Pa had brought with him disappeared after a few weeks and Marina took over the care of my baby sister. I couldn’t understand how the red, squalling thing which often smelt and diverted attention from me could possibly be a gift. Until one morning, when Alcyone – named after the second star of The Seven Sisters – smiled at me from her high chair over breakfast.

‘She knows who I am,’ I said in wonder to Marina, who was feeding her.

‘Of course she does, Maia, dear. You’re her big sister, the one she’ll look up to. It’ll be up to you to teach her lots of things that you know and she doesn’t.’

And as she grew, she became my shadow, following me everywhere, which pleased and irritated me in equal measure.

‘Maia, wait me!’ she’d demand loudly as she tottered along behind me.

Even though Ally – as I’d nicknamed her – had originally been an unwanted addition to my dreamlike existence at Atlantis, I could not have asked for a sweeter, more loveable companion. She rarely, if ever, cried and there were none of the temper-tantrums associated with toddlers of her age. With her tumbling red-gold curls and her big blue eyes, Ally had a natural charm that drew people to her, including our father. On the occasions Pa Salt was home from one of his long trips abroad, I’d watch how his eyes lit up when he saw her, in a way I was sure they didn’t for me. And whereas I was shy and reticent with strangers, Ally had an openness and a readiness to trust that endeared her to everyone.

She was also one of those children who seemed to excel at everything – particularly music, and any sport to do with water. I remember Pa teaching her to swim in our vast pool and, whereas I had struggled to stay afloat and hated being underwater, my little sister took to it like a mermaid. And while I couldn’t find my sea legs even on the Titan, Pa’s huge and beautiful ocean-going yacht, when we were at home Ally would beg him to take her out in the small Laser he kept moored on our private lakeside jetty. I’d crouch in the cramped stern of the boat while Pa and Ally took control as we sped across the glassy waters. Their joint passion for sailing bonded them in a way I felt I could never replicate.