In order to cheer Haruto up after his anime was such a flop, Itsuki gets him as well as Nayuta, Miyako and Chihiro together so they can play a TRPG. Itsuki usually plays girls in games, but for some reason, Chihiro makes out their character to be Itsuki’s character’s little sister, which leads to Nayuta and Miyako also making their characters sisters of the other two. Haruto becomes the GM and does whatever he wants.
Okay! Then I'm going to be 
Senpai's little sister, too!
Oki, dann wird meiner auch deine Imouto!
¡Bien! ¡Yo también seré
una hermana pequeña tuya!
¡Bien! ¡Yo también seré
una hermana pequeña tuya!
Dans ce cas, moi aussi,
je serai ta petite sœur !
Bene! Allora anch'io
sarò uno sorellina del senpai!
Certo! Então eu vou ser irmã
mais nova do Senpai também!
Тогда и я в сёстры записываюсь!