Anetomo

The protagonist who's always being teased by his sister ends up in the same school as her and their childhood friend, Shinoa, and he's being forced into their literature club. However the club is in danger if they dont win the next summer competition. Otherwise the club will be abolished. In exchange for making the protagonist do the work, the girls all help out by giving him some extra services. Meanwhile Eimi, the protagonist older sister, declared war against the other girls and wont let them have her brother. [Translated from the official website]