Ooooo my iPad deleted my response (it was longtoo)
I think this sounds great. At the very least, you can be a positive influence/ role model in her life . You can compliment her hair, and curly hair in front of her. You can show her that curly hair is beautiful.
Also, maybe you can buy a couple hair magazines, maybe one with kids styles in it - the kids /people in those mags are usually pretty/models.. And maybe that might spark her interest in wearing a curly style. Maybe pointout actresses her age (or young actresses) that have similar hair type?
Madison Pettis is young and super cute, her hair is like 3c curly I think?
Here is a link with her pictures and what shows she has been in
Madison Pettis - IMDb
If you ever get a chance to talk seriously with your nieces mom, tell her she is seriously damaging her daughters self esteem. Its not "just hair" , it is who she is! And who she is should be beautiful to her own mother.