It's sad that the protagonist was thinking it's a sign he will go blind... what a nightmare. Also her sister dared…
The fact that they make it funny / something to joke about when dealing with deceit and intentionally hurting and creating trauma for a person is NOT OK!
I have a feeling that it could be Mu Qing Ge her uncle Ye Xin.
Now this would be a big betrayal, since as far as I understand he was the one who taught her ... also, why did he have that Celestial Book of Fate in his painting? Can it be that he is an immortal/ god in disguise?