The Legend of the Empress's Sword

A girl finds herself in another world. Were her body, of a little girl, is perceived as cruel and selfish. She changes her new family's perspective of her, and they grow to love her. But her brother is going off to fight in the war, and she learns of the military plans. She knows it will get him killed, so she sends and anonymous letter to the crown prince. Reading it they eventually exchange letters throughout the war. Until he finds out...