As a punishment for his antics, this boy was forced by his sister to wear a chastity cage, to painfully keep his tiny useless boner inside it. He tries to resist her, but it's hopeless: she incessantly fucks him, her big dick bringing tears to his eyes, while he groans and cries like a little girl.
He begs her to stop, but she only gets harder. After being told that he is merely being used for what he's supposed to, he realizes that he has no other purpose for her than to serve and please big sister, by being her personal fucktoy.