Is it price, coverage, amount of data in plan?
What makes something best?
Personally I think few non-Americans realize how poorly our carriers look compared to elsewhere in the world. Since your sister is not here for years it would not be best to get involved with a regular carrier and their contracts. Prepaid would be better.
CDMA probably will not happen. US CDMA means carrier branded phones or no activation. (Although there are exceptions, working this out quickly is logistically difficult.)
I can only find one source on the radio... Even though it is an eBay ad, it strikes me as probably accurate given what it states:
Nokia Lumia 625 625H 8GB Yellow Unlocked Smartphone | eBay
GSM 850/900/1800/1900, UMTS 850/900/2100, LTE 800/1800/2600
Throw out the LTE as it must be FD (Field Division) for compliance in China and in the US we have TD (Time Division). The two are incompatible even if the frequency is the same.
That leaves us with GSM (2G) which is a match anywhere and UMTS (3G) which seems off for the US market.
2G or EDGE speeds are about all you will get out of the 625H on any carrier in the US including mvno (airtime resellers).
Best bet... Go with an AT&T mvno like Cricket and purchase a Lumia 635 from them. Good value in a phone and your sister will enjoy decent data speeds and a generous 3GB allowance each month for highest speed data without paying a whopping phone bill. And that will help a lot when trying to Skype call back home. Speaking of Skype, the international voice rates are pretty good. Throw some $$ into Skype and you have instant international calling and messaging without an expensive plan required. It is month to month service and can be terminated without a penalty.