This is an idiomatic expression, and there is only one correct way to say it. That would be: "My sister said it to his face." The alternative to saying something to a person's face is to say it "behind his back." There are hardly any other possible alternatives. To say, "My sister said it to his face" implies that the sister said something that might have been taken as an insult or a criticism and therefore required a certain degree of aggressiveness or anger or boldness or cruelty. She did not say it "behind his back" but "to his face." She faced him and spoke plainly and directly.