The brother and sister are always fighting, to the point where their parents are fed up with them. But in reality, all this sibling bickering is an act to deceive their parents. Their true relationship has long since crossed the line from siblings to lovers. So when their parents aren't looking, they'll sneak some affection, and when their parents go out, they have sex like there's no tomorrow! The thrill of not getting caught by their parents heightens their sensitivity and draws them deeper into this forbidden love.
