While St. Jude might be the best option if it would admit the OP's niece, there are charity care/financial assistance programs offered at a number of comprehensive cancer centers in the U.S. Here are details for Roswell Park in Buffalo, and Memorial Sloan Kettering in Manhattan:
https://www.roswellpark.org/become-p...stance-program
https://www.mskcc.org/insurance-assistance/assistance