How do you figure that? Charlotte herself said the letter was addressed to Augustin, and that seems to fit very well to me: Augustin was the one who sent her away when she was with him on mission and wrote to Maximilien that she was their greatest enemy and had not a drop of blood resembling theirs. We have no record of anything quite so virulent from Maximilien, so Charlotte's assertions that the brother she is writing to "hates" her would seem to fit better with Augustin even if we didn't know that the letter was addressed to him.