What can be the reason?

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However, Song Yan did not show the suspicions that she had on her face in front of Zheng Baozhi. Instead, she questioned, "Then what happened to your sister?" 

"I don't know," said Zheng Baozhi with a sigh. Very quietly she subtly added, "I am not sure what happened to her as we have already stopped contacting one another after she egged my daughter to go against me. I only received a letter from her where she swore that she wouldn't let me have it easy even after her death." 

"Do you have that letter?" Song Yan tilted her head as she added, "Maybe there might be some correlation between that letter and the ghost that is haunting you."