Sorairo

You start off as Kenji, a pretty ordinary kid (yes you start off as a kid) who’s visiting his aunt during the holidays with his father. There, he meets his cousin Ai, a quiet girl who is just one year younger than him. They were left together in the house so they went out to the nearby sea to play.
But because Ai was so quiet, Kenji decided to play alone in the sea while Ai looked at him from the beach. Going back home, Ai lied to her mother that they had fun together. This continued right up till the end of the summer holidays.
Feeling really guilty, Kenji decided to be more friendly towards her. The next year, he took the initiative and played with her. Another year passes by and he came over to her town for the holidays. They are already extremely good friends and look forward to each others company.
Then the bombshell was dropped. Their parents were getting married. But they were still young and didn’t really understand what was happening. The only thing they knew was that they are now siblings. Kenji now stays in the same town as Ai and lives together with her. New neighbors, new friends and new adventures await.

[From Accany's review]