I don't think Tia wants to kill her mother and brother, but rather destroy the connection between the Emperor and the two so that there is no chance Lawrence gets power. If Lawrence loses favour with the Emperor, he loses any clot the Emperor has along with it. The Empress obviously won't support him given that he is miralia son, and more importantly her children were killed for him to rise. Also, the Empress has clot of her own still that is why Tia chose to approach her. More importantly in this game of thrones, she is likely to favour Cedric over Lawrence, hence she enticed her with revenge. Remember in the beginning it was stated clearly that you either win or you die, no in between.
Tia isn't stupid enough to commit regencide, it will destroy all she has achieved. The Emperor was implied to have a weak heart when he was yelling Lawrence a new one about his kid trafficking friend, so she may very well provoke his death by natural causes.
Also those who dislike Tia for her plotting in the last few chapters seem to have forgotten what this story is about, or haven't been reading closely. Tia isn't going to change to a 'good' protag, she was a villian and returned as a villian. A few chapter back she clearly showed that she isn't in this to preserve her life, therefore she won't play it safe. She is pretty much resigned to the fact that she may very well die, and she is more than okay with that and is going all in.
Also Bill was a liability in many ways so she killed him. Her mother killed her family and framed others to get her hands on the Rozan fortune, not to mention the abuse she inflicted on her daughter. Her brother used her, killed his own son to frame her and pretty much had her become a living corpse in the previous Timeline while destroying the kingdom because of Paranoia. The Emperor killed his kids, the Empresses family, Cedrics parents and so many other messed up things. What goes around comes around.