Onee-chan Gokko Episode 1

In the distant future, on the outskirts left behind by the rapid concentration of advanced science and technology in urban areas, lies a remote mountain village known as Kagami Village.
“Behave yourself, or a Double will come for you.”
Such old folklore still lingers in this village.

Akane Higure, a middle school student living in Kagami Village, has been eagerly awaiting the return of her older sister, Sui Higure, who is finally being discharged from the hospital after a year.
However, not long after her sister comes home, Akane receives a single phone call.

“The girl at home with you… isn’t your sister.”