Based on the adult game by Moonstone Cherry.

Keiichi lives under the same roof as his five younger sisters and he had a problem… he couldn’t help but be turned on by his sisters developing bodies.

One summer vacation, their parents decided to go on an overseas trip, leaving Keiichi home alone with his sisters.
Ririna, his serious younger sister, took a strict attitude toward Keiichi, thinking that her sloppy and perverted brother would become more and more vulgar while her parents were away.
However, Ririna secretly masturbates, even while saying that she hates naughty things.
And Keiichi just so happened to know that...

(Source: Getchu, translated)