@Kingdo because his father is a selfish piece of crap who pushes him to pal around with shady people, and his mother has been teaching him since birth how to stab people in the back in order to become the next emperor.
Then again, his mother wasn't really that wrong, considering they emphasized in one of these chapters, that of all the candidates to become next emperor, the ones who don't win are in danger of having their whole families wiped out by the one who wins, and the fact that Cedric's parents were falsely accused and executed by the emperor, seems to indicate that is the truth. I am surprised Cedric survived.
That's why I say that Tia can't be too harsh on her mother. Tia was taught to backstab people, and her mother was probably taught that either by experience or by her own parents.
I'm not sure we ever learned what happened to Tia's mother after her half-brother became emperor. I wouldn't be surprised if he had her killed.