Betrothal by Fire.
It was an old tradition in many clans.
To understand it, one first needed to know about the system of clan head succession.
In the Mashyan World, where matriarchal systems were the norm, the first daughter would never become the next clan head. Instead, it was the second daughter who would take on that role. This only changed in special situations. If the first daughter had less cultivation potential than her junior sister, then she could become the clan head.
A somewhat confusing concept…
In the case of Mu Lingxi and Mu Lin, the first daughter, Mu Lingxi, had far more cultivation potential than her younger sister.
Being a prodigious talent, Mu Lingxi was seen as the clan's representative in the cultivation world. Her tasks included exploring ancient ruins, acquiring rare artifacts, and mastering forbidden techniques. She even served as an emissary in higher realms.