18+ Otaku no Kairaku

Japan has instituted a universal conscription policy for their military. With their parents inducted into the army, brother and sister Takumi and Kuriko are free to satisfy their forbidden lusts. Until their parents return without warning, bringing with them a temple girl, Qoodou, and a secret mission: Takumi must deflower Qoodou--or be drafted into the army himself and never see his beloved sister again! Takumi agrees, on one condition: that he be allowed to marry Kuriko when the mission is over...

(Source: MU)