You'd do anything to stay out of jail. But if that's the kind of pay the guard wanted, the perpetrator has to do his best. And so this guy fucked her well, fucked her in all positions, so that the guard herself has wanted to taste his cock. And the ending on her belly completed the payoff. All debts had been paid. Here comes the long-awaited freedom.
The sister is really uninhibited. Judging by the reaction of his brother and his unwillingness to break away from the game, his sister is already bored with him to say the least.