Suiren to Shion by Hibiki Works

The protagonist's parents send him to an isolated island - Minase, to stay with relatives during the summer holidays.
He had already been there only as a child; his cousins Suiren and Shion live there.
Suiren was beautiful as a child, was very kind to him and was like a real older sister to him.
Shion looked like a tomboy little sister she always followed him constantly.
The main character is interested in how they have changed after so many years.
Upon arriving at their house, he hears familiar voices ...