Though Cersei missed the obvious flaw in the logic - why would a man send his own mistress, of whom she thinks he is fond enough to make a viable hostage, to service his nephew? And if he does care about her, AND was magnimonious enough to share his own lover with the king unasked, does she really think there is ANYTHING she can do to stop him from getting back at Joffrey for the way he treated Tyrion's gift? Either way Cersei has overlooked this peculiar-to-the-show hole in the scenario. Ironically it's probably because she does not have a cock...