How does Ella break her obedience curse?

When Prince Char orders Ella to marry him she thinks about the terrible danger she might put him in if she does. Ella struggles with the words but she is finally able to refuse him. Ella is ordered to her room by her step sisters when she begins to shout her refusal over and over again, but she refuses them as well. Ella realizes that by refusing to marry Prince Char her curse was broken and she will be able to safely marry him after all.