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.
While we're at it, can I make it so that 
she's your character's little sister?
Imouto
Ya que estamos, ¿puedo hacer
que sea tu hermana pequeña?
Ya que estamos, ¿puedo hacer
que sea tu hermana pequeña?
– Je pourrais jouer ta petite sœur ?
– Ma petite sœur ?
Già che ci siamo,
posso farla essere la tua sorellina?
Aproveitando, posso fazer com que minha
personagem seja irmã mais nova da sua?
Раз уж такое дело,
можно, она твоей младшей сестрой будет?