Fang Yuyang and Fang Jintang witnessed a scene that Fang Yuxin also "saw." She was, after all, still worried about the father and son, so she quietly released her Spiritual Sense to follow behind them, ready to help in case of danger. However, she hadn't expected to "see" such a scene.
She was a Pure Wood Spirit Body, practicing a profoundly mysterious Immortal Path Technique, and was particularly sensitive to the presence of life force. Even though she wasn't physically present, only having released her Spiritual Sense, her perception was far more acute and deep than that of Fang Jintang and Fang Yuyang. The little girl must have had the Fire Ability, but clearly, she had awakened it out of desperation, unleashing an incredibly powerful Energy by burning her Life Force as the price for her and her mom's revenge.