Showing procedure eating peach at the beginning already a hint of what happen to them...
So... not all girls can eat peach and remain human, if they can't handle peach power, they become half human half shuuki like MC's sister and goverment throw them to that world to die, if they compatible with the peach power and didn't change into shuuki, they become demon corps...
Well, Goverments always fucked up in the head...