While their parents are away (capturing and training twins to sell as slaves in another story by the author), a brother or step brother pretends to break into their house, persuades his sister that he has tied himself and then enslaves and trains her for some period of time without revealing that it's him. Then he, as the anonymous slaver, leaves and starts to take advantage of the experience the sister has been through as himself.
Edit: I think I remember that the word summer was in the title or the author's name.