She had been given a 10-year visa, because one of her other sisters is married to an American, and she was there visiting them.
The 10 year visa was granted solely on the basis of a relative's marriage to an American citizen?
There are different kinds of visas. Anyone visiting the US (legally) must obtain one. Hers is probably an extended visiting visa, which does not allow her to live here, but allows her to come visit without having to apply for a visa each time.