Awesome! Your niece will benefit greatly from being with an open and honest person like yourself. And she will keep you young.
Your sister should feel grateful her daughter will be safe with family. She may be upset in the beginning, but hopefully she'll come to terms with it. She chose to raise her daughter in a closed minded environment, what did she think would come from that?

I think your niece will have a very good life. She seems to be a trooper. 👍

She is very fortunate to have you in her life, and I see it being a very positive thing for both of you.