Sukisugiru kara Kanojo Ijou no, Imouto to shite Aishite Kudasai.

I, Keita Majima, a normal high school student, was unexpectedly hired as part-timer at the game company of my dreams! My job was to be the "big brother for hire," here to help them create...... the ultimate sister character. It's the start of a comedy about a secret brother-sister contract!

(Source: ComicWalker, translated)