Amakara Twins ~Futanee to Issho~

The main character is Riku, who is back to his hometown. Since his parents have gone abroad on business, he starts to live at his uncle's house. After a five year interval, he is back to his hometown, Fukasawa Town, but there is nothing interesting.... At his uncle's house, there are girls, actually twins, Haruka and Natsumi. They are older than him and uni students at present. He is surprised to see them grown up. One day, his uncle has gone travelling with his wife for a while. This means there are only Riku and grown up twins at house.... His busy summer holiday starts now!

[From ErogeShop.com]