Yunnan sheng 雲南省 [Yunnan Province] in the Yuan dynasty has the full name of Yunnan dengchu xing zhongshu sheng 雲南等處行中書省, which is one of the ten xing zhongshusheng in Yuan China. It has a large territory which covers modern day Yunnan Province, Guizhou province, and parts of Tailand and Northen Myanmar. In the following Ming dynasty, its name changed to Yunnan dengchu chengxuan buzheng shisi 雲南等處承宣布政使司, which covers almost the same area. In the Qing dynasty when the story of Destiny of Rebirth is written, Yunnan province became one of the eighteen sheng 省 [province] of the Qing empire.
Yunnan Province in the Ming and Qing dynasties.
Yunnan fu 雲南府 [Yunnan Prefecture] was a secondary administrative unit located in Yunnan Province from 1381 to 1912. It was also called Kunming fu 昆明府 [Kunming Prefecture].
Yunnan Prefecture in Yunnan Province (1820).
Also, the author Chen Duansheng’s father was once Vice Prefect of the Prefecture of Lin’an 臨安 in Yunnan Province, and Chen may have stayed with his father when he took employment there. It is possible that Chen has an adequate amount of knowledge about the region of Yunnan. However, due to its geographical location being far distant from the central power of all dynasties of late imperial China, it is also possible that she set up the story in some place like Yunnan province to add some legendary flavor, especially considering the abnormal deeds done by the female protagonists.