I like the voice of the sister. I do think the entire game does not need to be voiced, it should be saved for the erotic scenes or emotionally important ones, just going around the school, however, does not need it. It just slows things down, we read faster than we speak so listening to the game just slows the pace. That's why I think it should be saved for important moments.
Think of it like a japanese game, you get fully voiced cutscenes, but text when moving about and talking during gameplay so it stays well paced.