俺の妹が最高のオカズだった

My very own little sister caught me masturbating to a little sister character in an erotic game. Then to my surprise, my cool, calm and collected little sister suddenly decided to help me masturbate... She even got her friends to masturbate me, including the most beautiful girl at school!? How will I be able to get a girlfriend if I can only cum with my own sister!? And… why is she still watching me masturbate!? (Source: France Shoin, translated)