Onii-chan Asobo

Koichi Akiyama, an ordinary 22-year-old game developer, has been living a boring life until one day his younger sister Ai paid him a visit. 
Soon after Ai moves in with Koichi, she claims that she wants to become a game developer too, so her brother has to teach her everything he knows. 
The only problem is that Ai wants to create Hentai games, which puts Koichi in a dubious position. 
From now on these two need to learn how to work together and explore all the difficulties of writing Hentai stories.

Steam