“We’re jumping in here?” I ask my sister, as we rush to jam fins on our feet and sling snorkels over salty ponytails. Nervousness lodges in my throat as I consider the blue deep that goes on forever under the catamaran’s rolling hull.

Her husband keeps the 40-foot boat a short distance from the waves breaking against the sharp cliffs. He’ll cut the engines just long enough for us to slip into the ocean, where we’ll swim a loop around this tiny atoll of the British Virgin Islands, before he returns to cut the engines again for the instant it takes to scoop us up from the sea.

She flashes a grin at me through her mask. Her smile is so like mine that people often think we’re twins, even though she’s older by exactly a year and a half and I’ve got a couple inches on her petite frame. Our minds often work as twins’ do; although we’ve been separated by landscapes and life choices for years now, she still sees into mine.

“This is my kind of adventure,” she says. “Think of it like dropping a steep line on your skis.” And she slides into the blue.

My sister and I grew up on the edge of the Southern California desert, made from equal parts dusty ponderosa forest and saltwater. The rugged San Bernardino Mountains were our regular playground, hiking and camping in our giant orange family tent, as much as our frequent trips to the coast where my dad had a sailboat in Dana Point when we were little.

I have vague memories, like a horizon veiled in sea mist, of my parents putting us to bed in the tiny forward berth on that boat and falling asleep to gentle rocking and the slap of sea against the bow, curled against my sister. I imagined we voyaged to distant harbors at night, my mom and dad unfurling the sails and navigating under bright stars undimmed by the choke of city lights, to arrive back at our own slip by the time I woke up in the morning.

My memories of sledding in the mountains that defined the skyline out our living room window are clear like a film reel. My sister appears right next to me, pawing through our cardboard box of rarely used snow gear in the hall closet in the never-ending search for matching gloves and that hat with the giant pompom that we both loved. She’s behind me, scream giggling in my ear as the plastic sled hurtles down the hill and kicks cold snow into my face, joy swelling like a balloon in my chest at the speed and the feel of the tiny crystals melting on my skin.

When I asked my dad, a lifelong product of orange groves and waves, why we never went skiing, he replied, only half joking, “You can stand in a cold shower and rip up $20 bills for the same effect.”

I finally learned to ski in the mountains of Montana, when my sister moved there for graduate school and I followed, living next door and wearing a path between our back patios. Snow edged out desert heat in my bones. It fell in my dreams and built up in drifts behind my eyes. But while I began to live for high snowy playgrounds, my sister slowly came to dread Missoula’s infamously gray cold seasons. She’d taken advantage of her San Diego undergrad university’s cheap boat rentals to learn to sail, and she escaped the winter months to hitchhike as crew on sailboats in tropical climes.

Eventually I left Missoula in search of bigger ranges and deeper powder. Where my sister got married and looked into sharing sailboats in equatorial harbors, I pared down so that most of my life fit in the back of my old Tacoma and went north to Revelstoke, B.C. After nearly a decade with only a fence between us, now there was an international border.

The stars swung slowly around the mast of my sister’s borrowed sailboat in Tonga. Her husband had gone to bed, leaving us up talking as we often did during that month that I joined them on their yearlong South Pacific sojourn.

Into the warm breeze that blew across our bare shoulders, my sister said, “I think I might be pregnant.”

“What! Really?” Words jumbled in my roiling tide of emotions. Surprise, delight, and something else I couldn’t quite identify. Suddenly it emerged clear: loss.

After articulating the first two sentiments accompanied by a hug, I took a deep breath. It was too hard to keep things from each other. “Our relationship will change,” I said. “There’s still so much we were going to do together.”

“I know,” she said, and we fell silent in the dark.

As I moved through my 20s, I became certain that I didn’t want children. When I have dreams of being pregnant, I wake up gasping in a visceral reaction. Many people would firmly place me under the “selfish” umbrella and leave me there. I’d like to redefine it, but I’m still thinking about what other words fit, since we don’t have too many for women who choose not to become mothers.

Over the next few years, a space began to open between my sister and me, magnifying the literal landscapes between us. While I explored headlong deeper into the mountains on a schedule all my own — ski touring all day without having to report home, heading up to the hill for a powder morning that doesn’t require finding child care — she was neck deep in the hard parts of raising two children under 5, the parts that no one seems to tell you about before you actually have babies, and the parts that are lost in translation to childless people who can’t quite grasp the profound balance between the hard and the joy. And as I turned to the mountains, she headed more and more to the sea with her children in tow.

We stopped understanding each other’s lives on a daily basis, and we felt it like the gaping hole of a wound. But neither of us was willing to give up any of our landscape for the other’s.

When she asked me last November to come sailing with her family in the British Virgin Islands, I immediately declined. It was too far from Revelstoke, whose nearest airport is two and a half hours away and typically requires about eight connecting flights to get where you’re going. The trip would be too expensive, too time consuming. And it wasn’t my kind of thing anyway.

But then I remembered the forward berth on my dad’s sailboat.

Which is how I found myself on a catamaran in the Caribbean, following my sister into the deep on her terms. And in swimming with my nephew in calm bays where he’d spot turtles before any of the rest of us, in rocking my niece to sleep to the rhythm of the wind, in watching my sister navigate the ocean — all the miles, hours and convoluted routes to join them from a remote Canadian mountain town faded into insignificance, transmuted to the nexus between mountain and ocean where I grew up with the sister whose smile is mine. The only sad thought that I kept pushing to the back of my mind: we can’t do this all the time to re-find each other’s daily.

My sister calls me her “ghost ship,” that shade of a possible life before certain decisions are made, that scuds off in a blurry dream on a different tack than the one you’ve taken. Sometimes it’s visible between the waves. Sometimes it’s veiled in sea mist. But you always know it’s out there, sailing parallel for a different shore.

Cassidy Randall is a freelance writer covering public land, the environment and adventure.