Piano no Mori no Mankai no Shita

Ryutaro cares about his younger sister Sakuno more than anything else in the world. But, she has an incurable sickness eating away at her. When the doctor announces that she only has a week left to live, he takes her to a sanitarium with some mysterious tickets, seeing it as her last hope. Upon arriving at the secluded building, tucked away in the mountains, they are greeted up by Konohana, the nurse in charge and the only human nearby. With only seven days left, just what can happen? Just what can be done?

[From a review by Choux]