The Sect Leader of the Luo River Sect, also the Heavenly Emperor of the Luo River Heaven, had a pair of twin daughters. One of them was the Saint Girl of the Luo River Sect, and the other was the soul who possessed Mu Yi Yi.
When the Heavenly Emperor of the Luo River Heaven gave birth, the younger twin's chances of surviving were incredibly low due to her physical disabilities. For that reason, the Heavenly Emperor of the Luo River Heaven forcibly extracted her youngest daughter's soul before letting it take shelter in her eldest daughter's body. Hence, many people thought she only had one daughter.
This was, naturally, just a temporary measure. After all, the Heavenly Emperor of the Luo River Heaven loved both of her daughters equally; how could she allow her youngest daughter to take possession of her eldest daughter's body? Since long ago, she had planned to look for a suitable body for her youngest daughter.