There's a lot of mention of Tibetan culture (obviously, since the story is set in Tibet), but Rati is simply described as being from some neighbouring ethnic group. Looking at the clothes she and her father wear, along with the architecture in chapter 16's flashback, I'm guessing she's Tujia. That said, I'm no expert (I only looked on wikipedia for possibilities for a few minutes). However, I'm not sure if the Tujia would have counted as "neighbouring" during the 18th century. Opinions?

Edit: The end of volume 5 finally answered this one. Rati is of a Liangshan subgroup of the Yi. Their clothing is pretty elaborate and varied (why I didn't consider the possibility).