The letter that finally ended Thomas's uncertain hedging arrived on an ordinary evening in early winter, and this time there was no comfortable explanation waiting at the edges of it.
Your father's talking about retiring next spring, his mother had written, now that the twins next door are old enough he doesn't feel obligated to keep the yard looking presentable for the neighborhood anymore. Can you believe it's been almost fifteen years since your sister was born? I still remember bringing her home from the hospital like it was yesterday.
Thomas read the passage three times before the wrongness fully landed, cold and absolute in a way none of the previous discrepancies had managed.
He did not have a sister.
He had turned the lake trip aside as ordinary forgetfulness. He had turned the neighbors aside the same way, a detail small enough to have simply faded from disuse. A sibling was not a detail. A sibling was the kind of fact a person carried in the very structure of their childhood, present or absent in every memory of every holiday, every family dinner, every ordinary year of growing up — and Thomas possessed, searching through his own memory with a desperate, clawing thoroughness he hadn't allowed himself before, absolutely nothing. No name. No face. No single fragment of a childhood shared with anyone but his parents.
He found Victoria at the hearth, and the look on his face alone brought her instantly to her feet.
"Thomas."
"I have a sister." His voice came out strange, hollow, barely his own. "Fifteen years old, apparently, born while I would have been — nine, ten years old myself, old enough that I should remember her birth as one of the largest events of my whole childhood. I don't remember her, Victoria. I don't remember anything about her at all. Not a single day."
Victoria's face had gone pale, her own composure visibly straining against the weight of what he was describing. "This isn't like the others. This isn't a small missing detail."
"No. This is a person. A whole person, apparently real enough that my mother writes about her as casually as she writes about the weather, and I have absolutely nothing where the memory of her should be." Thomas sat heavily, the letter still trembling faintly in his hand. "If Genoa's clerk sketching a false map could produce a report someone believed, I don't understand how a mind can hold a gap this large without the gap itself feeling like anything at all. I should feel her absence, Victoria. I should feel like something's missing. I don't. I feel like I've always been exactly what I am right now — an only child, whole and complete, with no sister-shaped hole anywhere in me at all."
Victoria was quiet a long moment, and when she finally spoke, her voice carried a fear Thomas hadn't heard from her since the very first night he'd shown her the falling number.
"What if she was never there to be missing, Thomas? What if this isn't a memory failing to hold something that was always true. What if the thing itself has genuinely, retroactively become true, in the version of the world your mother's currently living inside — a sister who was always there, from her perspective, because your own presence here has reshaped enough of the past that an entirely new person now occupies a space that used to be empty?"
The theory landed with a weight that made Thomas's vision swim, the cold, expansive horror of it considerably larger than anything he'd let himself imagine even in his worst private hours.
"You think I've changed history so thoroughly that an entire person exists now who never existed before. That somewhere, in the world I left, a fifteen-year-old girl is alive today, with a whole childhood and a whole life, because of something I did here — something that rippled backward far enough to write her into existence."
"I don't know, Thomas. I only know the alternative — that your own mind is simply failing, piece by piece, in some ordinary and comprehensible way — frightens me rather less than the possibility that you've been quietly, unknowingly rewriting an entire family you'll never get to properly meet." Victoria took his shaking hands in her own, steadying them the way she'd steadied so much else across their difficult years together. "Whatever it is, we record it. Same as always. And we keep living the days we still have, because I don't believe either of us has the power to stop whatever this actually is, only the choice of how honestly we face it while it continues."
Thomas nodded, though the cold, hollow shock of it didn't fully leave him for the rest of that long, sleepless night — sitting alone with the Codex's growing quire of wrongnesses, adding a sister's name he'd have to ask his own mother for, in a letter he wasn't at all certain he had the courage yet to send.