This is starting to feel more like a prequel to that hentai series from Takei Ooki, "Watashi no onii-chan" which after shit tons of incest the imouto eventually mind-breaks her onii-chan's fiancee into accepting imouto as onii-chan's true wife and them (fiancee and onii-chan) getting married in name only.
if anyone wants a game with a cute and funny little sister, then you should play "living with sister: monchrome fantasy". dont even need to google the full name cause i played the shit out of it