HI BATHONY,
If feeling very concerned about your niece, I would phone child welfare/ child protection. So they can investigate and provide ongoing support etc for the family and monitor the situation. Is it not possible to try at maybe resolving any differences with your brother, or at least visit and be a part of your niece's life? Very difficult circumstances, has this triggered memories for yourself?
Are you currently in counselling? Maybe a option depending on how you feel about it. Please don't blame yourself, or think your bad or anything not your fault what happened to you. Sometimes in forums it might take a little while for replies, everybody cares so much, your not alone.
TAKE CARE
KATE