Yes. I think you just spoke to my heart.
Her father is gone...and can’t do any more harm...
God willing...let there be no more poison.
That time is over...
Any physical, spiritual, and emotional harm your brother inflicted on you has been taken up by Christ through His redemptive suffering. It does not own you anymore. Extend to your niece the mercy Christ showed you.
I think meeting her with a trusted friend or your pastor would be best. You can decide after such a meeting whether you think she can handle the truth of her bio-dad. Ask her permission to contact her family and get their take on all this.