Heavenly Saint Cult was a large family, and the 'Si' in Si Yunxiang's surname meant to manage, to control. Ever since Heavenly Saint Cult was founded, Si Family was in charge of managing the cult's skills, inheritance, wealth, and propagation.
Because Si Family was extremely important, the saintess of Heavenly Devil Cult would most often come from Si Family. She would be its head, and to some extent, the importance of Si Family would even be higher than that of the cult master sacred teacher.
Granny Si was like that before, and Si Yunxiang was the same after.