MKON-148 I, the gloomy guy, get used as material for my sisters' punishment game and have non-stop in-the-vagina harem sex!

She used to be a cute little sister, but lately she says things like 'Ew, gross, big brother' and 'Don't do laundry with me' every time she opens her mouth. One day, my sister came home with a friend. 'Don't leave your room!' I was told, so I stayed quietly in my room. Then came the sound of someone rushing upstairs. The next moment, the door was flung open and my sister showed her panties. "Die!" she said before quickly leaving. What the hell was that?? It seems I'm being used as the punishment game master, and the difficulty is gradually increasing. I finally had sex and came inside with everyone in the harem!