The Thief of Forgotten Stories

The moment Iris stepped into the Archive, gravity apparently decided to take a holiday.
Books floated through the air. Staircases rearranged themselves, and tiny golden creatures carried letters between shelves.
"Okay," Iris whispered. "This is officially better than school."
Her brother followed her through the mirror.
"Don't touch anything."
Naturally, Iris touched the nearest book.
It sneezed.
"RUDE!" the book shouted.
Iris jumped.
"YOUR BOOKS CAN TALK?"
"Only the dramatic ones," her brother replied.
Before she could respond, a small creature wearing spectacles appeared.
"Visitors!" it squeaked. "How terribly inconvenient!"
It introduced itself as Pip, the Archive's assistant librarian.
"We're looking for a stolen story," Iris explained.
Pip's expression changed.
"Oh dear. Another one?"
"ANOTHER ONE?"
Pip led them towards a magnificent golden door.
"Someone has been stealing stories for weeks. Entire memories are disappearing. Friendships, adventures, even people's names!"
Iris frowned.
"Why would anyone steal memories?"
"Because," said a voice behind them, "some stories are worth rewriting."
They turned.
A girl with silver hair stood on a floating staircase, holding a glowing book.
Iris recognised the title immediately.
THE GIRL WHO STOLE HER OWN STORY.
"Give that back!" Iris demanded.
The girl smiled.
"Are you sure you want it?"
"Obviously!"
"Then tell me something, Iris. What's your earliest memory?"
Iris opened her mouth.
Nothing came.
She tried remembering her fifth birthday.
Her first day at school.
Her mother's face.
Everything suddenly felt strangely distant.
The silver-haired girl descended the staircase.
"Your memories aren't disappearing because someone stole them."
"Then why?"
"Because they were never yours."
Iris felt the room spin.
Her brother stepped forward.
"That's enough, Lyra."
Iris turned sharply.
"You KNOW her?"
Lyra laughed.
"Oh, he knows me very well."
She opened the glowing book.
Golden letters rose from its pages, forming a sentence in the air.
IRIS BENNETT WAS CREATED TO PROTECT A SECRET.
Iris stared at the words.
"What secret?"
Lyra's smile vanished.
"The one your brother has been hiding."
Suddenly, every book in the Archive began whispering the same name.
"Evelyn... Evelyn... Evelyn..."
Iris looked at her brother.
"Who's Evelyn?"
He went completely still.
Then, from somewhere behind the golden door, a familiar voice answered.
"That would be me."
The door slowly opened.
Standing inside was a girl who looked exactly like Iris.
Except she was seventeen.
And she was holding a photograph of their family.
With Iris missing from it.
End of chapter 3.