Based on the erotic game by Pita Fetish!.

To overcome his attraction for his younger sister Hana, Yuuichi Mitsui made sure to enroll at a university far away from his home. Unfortunately, being separated from her only strengthened his feelings. He continues to feel guilty for harboring illicit thoughts when suddenly, Hana shows up at his doorstep on a summer morning, looking more beautiful than ever.

However, despite her physical maturity, her intellect is that of a child. Consequently, Yuuichi tries to hold back from laying a hand on his innocent sister. But it is easier said than done, especially when Hana's ignorance—combined with her innocence—makes her a woman that Yuuichi can't help but desire.

(Source: MAL Rewrite)