I can understand the feelings of guilt. You did to your brother things that other people did to you that hurt you. The guilt is perfectly understandable. But it is time to let it go. I would recommend talking about this to your therapist. She will help you get to understand why the little girl did what she did to her brother and she will help you see that it wasn't her fault. You were a child who was abused then forced into the role of a parent. Is it not surprising that you made a mistake? Please talk to your therapist about this, you deserve some peace of mind.
Carrie