なつかげ

Left to take care of the house and his little sister when his parents go overseas for work, the protagonist Asaka Hayato, decides to take his sister on a much needed vacation to a quiet resort town to escape the summer heat. Their friends, sensing danger, plan to join them on their trip. The sibling vacation becomes a five person one when their childhood friends -two sisters, and one bad influence- tag along.

The danger his friends feared was Hayato making a move on his little sister Miki if they traveled by themselves. They decided letting someone with such a painfully obvious sister complex take a trip alone with his sibling was much too risky. But Hayato has no self awareness, and while he can tell Miki has a slight brother complex, he thinks of himself as just a kind older brother.