Hopefully the rain predictions won't come to pass, but even light rain isn't too much of an impediment if you have a lightweight poncho to toss on. As for the sister's problem, you might be right. The area is traditionally Tibetan but the JZG area has never seen problems like up in Ngawa or Xiahe or Rebkong, plus many of the JZG locals are engaged in the tourist industry. At any rate, the Han influx of residents has been considerable so you have to work a bit harder or go a bit farther away from the Park to see larger conglomerations of Tibetans.