The rules differ from airline to airline, however most airlines will have rules about unaccompanied minors, and these rules are always available on the airlines' websites. Note that some airlines may charge an extra fee for this service.
As your niece has a green card, I assume she either have a passport from another country, a US re-entry permit or a refugee traveller's document. I assume that the visa is stamped into that document.
Apart from the passport/visa, as the child will be travelling outside the country, some airlines require a notarised letter from the child's parents indicating their agreement for the child to travel without them (called parental consent letter). Some airlines will even require both parents (or guardians) to sign such letters.
Finally, note that often airlines will not sell tickets for unaccompanied minors via their websites - and you are required to either by the ticket on the phone or at the airline's office.
There are no other requirements other that these. I had my nephew from abroad visit me in the UK - and had no issues whatsoever. I do recommend talking to an airline directly if you have any concerns.