We're told there are much fewer Dixingren than haixingren, so I'd assume the population of Dixing is more similar to that of Dragon City than of Haixing. Powers are sort of hereditary (since the Chu are a family of puppetmasters), so uh I guess if mom came from a family with gourd-related powers...

Sha Ya's an orphan? She's old enough that she'd plausibly have moved out and settled independently/gone to faff about on the other side of the city, assuming dixingren have similar "life cycles" to us.