Baka na Imouto o Rikou ni Suru no wa Ore no ×× dake na Ken ni tsuite

Based on the erotic game by Pita Fetish!.

The Mitsui family desperately wanted a girl, so when Mitsui Hana was born, her parents spoiled her very much. So when Mitsui Hana big became a very stupid person. Things to remember will forget when walking 3 steps, and he can not read the atmosphere, and also he does not understand a button that is more than 3 buttons. Mitsui Yuichi was assigned to take care of Hana's younger sister as long as her parents went abroad. And Yuichi must be able to face this cute and stupid sister.

(Source: Nekopoi and Has Been Translated)