Ani to Imouto no Shitai Shitai Shitai Koto

Kouta Moriyama is an unsuccessful adult manga artist. Due to his lack of popularity, he was on the verge of quitting. But one day, he finds success with the vividly realistic portrayal of a charming heroine. The source for this sudden change in style is his little sister Shizuku...?!
(Source: Kadokawa, translated)