I'm sorry to hear the sad news about your niece. I suggest that someone contact St. Jude immediately:
https://www.stjude.org/
(Click on "Patient Referral.")
If you, or any of your niece's other relatives, are U.S. citizens residing in the U.S., contacting your Congressional representatives might help in getting the girl admitted into the U.S. for treatment. And the folks at St. Jude might have some experience/advice about how to get the necessary visa.
Information about obtaining a B2 visa for medical treatment provided by the U.S Embassy in the Dominican Republic:
https://do.usembassy.gov/go-united-s...cal-treatment/
I wish your niece and her parents the best of luck.