Thanks for the chapter!
It's mentioned several times in the LN that MC has a little sister and has been traumatized by her. Seems she's a yandere brocon although other than calming himself down by reminding himself she's in Japan and occasionally wondering if she's alright he never reminisces about how she traumatized him. So could be anything from extreme hazardous cooking, to S&M, to who knows what. Luckily for him she's still in Japan. So the dark haired girl saying Oni-sama over and over is his little sister.