Tsun na Imouto o Dere Otosu!! Futari de Kurasu Sanjuu Nichi Kan [Studio Neko Kick]

Description: Due to his parents’ sudden trip abroad, the protagonist has to live alone with his rebellious younger sister for a month.
One night, he peeks into his sister’s room and finds her struggling to masturbate for the first time.
As a brother, he decides to develop his sister’s body in the middle of the night in order to release her sexual desire. The naughty 30 days with his sister begins!