Daughter/Sister of Mine was a part of a criminal family known as the Family of Blood. She appeared to be the youngest memeber, because she possessed the body of Lucy Cartwright, a little girl. When she was caught by the Doctor after her family tried to take his life essenece, he trapped her inside every mirror in existance, visiting her once a year, every year. (DW: Human Nature / The Family of Blood)
Behind the Scenes
In the book Martha in the Mirror the Doctor mentions "A Girl With a Red Balloon" , Possibly meaning that she was haunting the castle by being in the mirrors.