Aphasia

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. With her harmless appearence she was able to get into places, like the party and the Farringham School for Boys. 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.