By the end of the book, I feel that Briony has not completely atoned for her childhood 'crime'. Although I do believe that stories have to power to change things and make bad things right, I also believe that this only extends to a certain point.
I think that Briony's character expresses a great deal of guilt throughout the third part of the book, and I do believe that part of that is alleviated through the fictitious moments of happiness that she creates for her sister and Robbie. I also think that the character's very public admission of her 'crime' in the form of a novel helps to sponge away some of the guilt as well. However, I do not think that her guilt could ever be erased or that her 'crime' could ever be corrected or forgiven. It can be understood, because of her age at the time. Although I just do not believe that even her best efforts to punish herself or correct the past can make the pain that her sister and especially Robbie had to endure. Nothing short of going back in time and making it so that it never happened could fully make up for what she did, but it is endearing that she immortalized the ones she hurt most in her novel, giving them the happy ending that they so deserved, but were denied due to her actions of youthful ignorance.