Onee-chan wa Uchuu.

Our protagonist loves visiting his grandparents’ house during the summer. The reason is simple: he's in love with his cousin Aoi who lives with them.
This summer, he finally gathers the courage and confesses to her. What follows is a hot and sticky summer filled with great memories of his “older sister”, as he often refers to Aoi.
(Source: 2DMarket)