My sister Sera was twelve standard years old when our parents confined her to our family habitat. They kept her there for over a year, and then they died far from home (expedition, landslide). I’ll never know if they were seeking a cure for Sera, or a way to protect the world from her.

They must have died mid-morning, but I didn’t learn about it until I left Sera’s side to make lunch. The habitat’s weak AI played their pre-recorded message once I was alone.

“Alene,” our mother said, her arms around father. “If you’re seeing this recording, your dad and I are dead.”

“We’re sorry to leave you and your sister alone,” our father said. “We never wanted to be parted from you.” I’d barely seen them over the last year.

“But we know you can take care of the habitat,” mother went on. “We know you can care for Sera. Like I told you when she was a baby: you have to look out for her.”

The lines at the edge of father’s eyes tightened. “Yes,” he said, his tone flat. “Look out for her. But do not let her out of the habitat. She’ll corrupt the planet. You must keep her inside.”

Mother squeezed his shoulder. “We love you girls. We have an obligation to protect you, and to protect all of Lilit.”

They said more, but it washed over me. I felt hollow. Their deaths hadn’t touched me yet. They were gone, and Sera and I were the only humans left on the planet.

They’d made me my sister’s jailer.

• • • •

I was six months old when we landed on Lilit. Sera was born three years later. Now I wonder: was I born too soon? Or was she born too late?

The day of her birth, I’d been outside playing among the great, flexible trees, chasing ineffectually after the sloots that weaved and darted, always just outside my reach. My parents rarely let me play unsupervised, fearing the damage I might do to the ecosystem, but that day they were occupied, and I was allowed to run free. My delight was broken when I heard the screams.

I rushed inside, but the screams had stopped and mother was already feeding the red-faced infant.

“The baby was crying,” I said, worried.

“Yes, dear,” mother said. “Your sister was crying, but that means she was breathing well. It’s a good sign.”

“Good,” I said, and leaned up to watch this tiny person, her face furrowed in concentration. Mother patted the seat beside her, and I climbed up to join them.

“There’s only the four of us on this planet, Alene,” mother told me, rocking Sera gently. “You must look out for your sister. Promise me you will.”

I promised, certain that nothing would separate us.

• • • •

Our parents were biologists, and they believed in leaving no trace. They thought they could somehow call a place home yet not shape it, or leave only trace footprints. They’d converted our ship to a dwelling, sealed the food we grew inside to minimize the chance it could pollinate with the strangely Terran-like flora and fauna of Lilit. They thought they could study the ecosystem while remaining apart from it, and they preached the importance of honoring this world, our adoptive home. Sera’s native world.

Two days after her third birthday, Sera began to flower. Tiny, delicate buds, purple shot through with gold, sprouted from her arms, shoulders, encircled her head like a garland. We were playing outside the habitat, chasing sloots until they got tired of the game and glided up to the treetops. Sera was trying to catch her breath, panting and laughing, when the buds emerged. I watched, captivated and jealous, as the flowers swayed with her every movement. I didn’t think to worry until I saw father’s expression. He scooped Sera up and ran inside. Sera’s shock gave way to screams.

He took her to the lab, where mother was analyzing samples, and sat Sera down. She sobbed. I sank to the ground beside her, wrapped her in my arms. Over her cries I couldn’t hear our parents’ conversation, but it was quiet, urgent, bordering on panicked.

They ran many tests on Sera but could not detect a cause. She seemed perfectly healthy and happy, though she hated being cooped up while they studied her. Maybe if they’d had access to a full suite of medical tech they would have quickly discovered the cause. But they only had what we’d been able to bring along. My father, who usually loved puzzles, ranted about the limitations of “this backwater tech.”

After a week confined to the habitat, Sera shed her flowers. The petals drifted in the circulated air, settled in clumps across the lab. Still without answers, our parents eventually let her back outside as they studied, tested, hypothesized.

For years, it went on this way. Sera would bloom every few months, and at the first signs of budding, she’d be confined inside the habitat until the flowers fell away. Then we’d go on, Sera’s condition mysterious and banal.

• • • •

Everything changed when Sera was twelve. The two of us were two kilometers from home, charting the growth patterns of local flora when she blossomed again. This time, though, the buds were different: each bulb was thin, iridescent as a soap bubble. We marveled, caught up for a moment in the uncanny beauty.

Wind stirred the trees, popping the bulbs. Hundreds of filaments flowed from my sister, lifted by the breeze, each delicate strand shining whenever they flowed through a patch of sunlight, then disappearing in shadow.

We stood transfixed, not yet understanding that everything had changed, knowing only that we’d witnessed something strange and beautiful.

Fear came later, as we trudged back home.

“We don’t have to tell them,” I said, thinking of our parents’ reaction.

Sera, brave or naïve, simply shrugged. “They’ll find out sooner or later.”

That’s how her captivity began, how our parents’ tests grew more urgent, their hushed conversations more strident. They hoped for an answer, for a cure. They might as well have hoped for a treatment for humanity.

Weeks passed, then months. I was shocked when Sera didn’t rage, didn’t beg, didn’t advocate for herself. She withdrew instead, spending hours staring at nothing, her thoughts unfathomable to me.

Our parents took refuge in research and my sister went somewhere untouchable. I was fifteen, furious. I screamed, ranted, made demands. Refused to leave the habitat, to help mother and father in any way. I wanted to sit with my sister, to be there for her, but she drew back from my anger as much as from our parents’ cloak of dispassion.

• • • •

Our parents sat me down a few months later. They’d discovered what was happening to Sera. She was not entirely human, father said, summoning a gentleness I thought he’d lost, and that I’d only appreciate much later. The local fauna, the tree-grazing thii with their downy fur; the graceful, predatory saar; and my sister: they all had three parents. The third parent, mother explained, had some properties of a virus and others of a prion, but didn’t really have much in common with either. It influenced (they were careful not to say “infected”) the genetics of every creature, every plant that germinated on Lilit. Its nature changed them, and their existence spread it. Our parents had ensured I had a good background in the sciences, but I struggled to make sense of this. I think mother and father did too. Perhaps they were so long in solving the riddle because they had trouble accepting that the alien biologies of Lilit could be close enough to our own to allow such a thing. Years later in their notes I would find the following in my father’s blocky handwriting: Sera’s DNA sequence has been altered. She would pass the contamination down to her children as surely as she would her mitochondrial DNA.

They planned to keep Sera confined to the habitat until they had a cure. They didn’t say it quite that bluntly, of course, either out of respect for my loyalty to Sera, or fearing that I’d once again scream myself hoarse.

They told me all this privately, as though I would be keeper of their secrets. “We don’t want to upset your sister,” my mother said.

“All this time,” I said. Horror had taken root in my gut. They were trying to fix a choice—I could not bring myself to call it, or even think of it, as a mistake—they had made twelve-plus years ago. “All these months, these years, and it was always too late.”

“We’ll find a way to set things right,” father insisted, as though my sister was a problem to be solved.

“‘Set things right’.” I felt venomous. “When are you going to stop punishing Sera for your choices?”

“That’s what we’re trying—” mother began. I fled the room.

• • • •

I don’t know what they’d planned to tell Sera, but it hardly mattered, since the first time we were alone I told her about their discovery. We didn’t keep secrets from each other, and I believed that united us.

Sera stared at me, her eyes moving back and forth across the features of my face. Then she said, “Oh. They think I’m a danger to Lilit,” and fell silent.

My brilliant, blooming sister wasn’t made to be contained. Buds still rose from her skin, but they were dull, shriveled things. Her skin grew ever more ashen and her hair came away in clumps no matter how carefully I brushed it.

Hatred burned through me like a fever. I told our parents they were cruel beyond all measure, caring for Lilit and not their own daughter. Now, I remember how they’d loved to watch us play, how they took us with them as they explored and cataloged the world, as they followed the migrations patterns of the thii as they grazed from the swaying treetops and taught us how to avoid the attention of the nocturnal saar. Now I know it must have hurt them deeply to lock her away from all of that, to know their that choices had changed Lilit and Sera forever before she was even born. But they didn’t know how to show me their pain, and I didn’t want to see it.

Oh, the damage we cause by doing what we think is right.

Consider our family, such as it was, in those long months: Sera’s withdrawal was mitigated only by the habitat’s garden, where she’d spend hours, tending to loam and branch, root and stem. I dreamed of escape, learned everything I could about the technology of our habitat, and planned ways I might convert it back into a ship, take my sister far from Lilit, far from our parents. And our parents, alternately sequestered in the lab or journeying far and wide, used the habitat’s AI to keep us locked inside.

So eager for a cure, they couldn’t see the real sickness which had taken root in all of us.

• • • •

To the very end, mother and father believed that they could call back the change they’d wrought, and if that proved impossible, that it could be contained forever. That was their wish, their only will and testament, their sole demand: be my sister’s captor.

I thought they were fools, but now I think they were merely desperate. I hated them as intensely as any adolescent girl has ever hated her parents, and with better reason than most. And I adored my sister. Yet they’d somehow believed I would do as they bid. Maybe they needed to believe it.

When I told Sera they were gone, she wept, for she had no hatred, only pain and grief and nowhere to put them. But I couldn’t cry. Tears were inadequate for the depth of my loathing and love, the knowledge that I’d never again express either truth to them.

I never played their message for Sera, couldn’t force myself to inflict our parent’s cruel words on her. It was the first and last secret I kept from her.

“Do you want to go outside?” I asked her.

“Mom and Dad wouldn’t want me to,” she said, staring at her hands. “I’ll change the world. Hurt it.”

“No,” I said. “Sera, Lilit changed the first time you flowered, and again when the flowers pollinated. They changed it the moment they landed. Changed it by making a life here.” How close I’d come to saying making life.

“But we might hurt them,” Sera said. “The thii and the tiny sloots, and all the world.” She was kinder than me, thinking of everything but herself.

“We could leave,” I said. “I’ve been studying. We could turn the habitat back into a ship, and we could find someplace else.”

“And we would change that place. I would change it.”

I considered this.

“There are no good choices,” I said. “Wherever we go, our problems come with us.”

“I didn’t want this,” she said. I brushed away her tears with my thumb. They smelled like the trees after rain.

“I know you didn’t. They chose for us,” I said. “Now we have to choose, as best we can.”

She sniffled, then grew silent.

“I can feel the fields calling to me,” she said at last. “This place is my home.”

Her home, but never mine. Together, we walked out into the open fields. Two sloots chased each other in elaborate patterns across the grass. Sera laughed in delight, chased after them. As always, they let her almost catch them. Within hours, she was blooming again, the delicate buds larger and more vibrant than I remembered. By then, I had already pulled the exterior doors off our habitat. I was done with barriers.

Lilit would change. Sera would change it and be changed by it. And I, forever a stranger here, would watch Sera bloom.