What jeni says, suggests to me that it was the parents who needed sleep "therapy", not the little girl. Maybe they weren't getting any sleep because of the little girl's problems. If they have two other children, they would need their sleep.
It would help if the OP would come back and explain further. I find it hard to believe parents would lock a child in a bedroom for trivial reasons.
How do I help my sister to stop feeling like a victim?


...but could be something else
...that the child themselves may not be overtly aware of?