Your sister's daugter can apply for student visa to study for two years during which your sister give your gardianship. after two years, after you are a citizen, you can apply green card for her before she is ?? year old( I forgot the age). I just read a book regarding this and it is the same case happened in the book. The book is called "My experience as a lawyer in the U.S.A".
It just you never know what would happens. She might not be as expected as you or your sister expected.
It just you never know what would happens. She might not be as expected as you or your sister expected.