Trinoline

Shun Nanami's sister Shirone drowned at sea one hot summer's day. He has never forgotten her, and carries the weight of it into every summer since. Years later, following a piano melody Shirone always loved, he finds the source — and standing there is someone who looks exactly like Shirone. Not his sister. An android named TRINO, built in her image. She calls him Onii-chan and asks him to take care of her. He does not know what that means yet, but he is listening.