The Sweet Ghost Girl is one of the "Three Souls" or Ghost Children trapped inside a wall by the Other Mother. She is Wybie's great-aunt and his grandmother's sister. A doll that resembles her can be seen in the beginning of the film before being converted to a doll resembling Coraline. So it's implied that she was the most recent (and final) victim for the Other Mother's grasp before Coraline. Her real name is a mystery since she doesn't remember it.
It is likely that the doll resembling Coraline is over half a century old, as the Other Mother might have converted it after the Sweet Ghost Girl fell victim to her, and left it with Wybie's grandmother all that time before Wybie gave it to Coraline.