As the younger sister of the four spiritual roots, Gu Xunyin can never compare to her older sister, Gu Moli.
This is a world where immortality is possible. Gu Moli obviously wasn't going to tell her the way to immortality.
Gu Xunyin overheard this secret while working by chance, and followed Gu Moli and her father to the sect.
From then on, Gu Xunyin began to cultivate demons in secret.