well i usually get confused if someone asks like this, bcs for me from this moment (ch 135) until ending is very tense so i think maybe you want to read this:
https://www.novelupdatesforum.com/threads/death-is-the-only-ending-for-the-villain.97486/
you can start from "spoiler: volume III: chapter 14"
It’s been a while since I read the novel so I don’t remember details but basically they confirm it’s the “real” sister and penelope is just about done with everything and everyone. When her coming of age party comes along she notices that the “real” sisters wine cup is laced with poison but then she realizes that if she got poisoned she would be blamed so she drinks the cup herself and Callisto goes ballistic
Ladies and gentlemen, I formally request everyone to spoil me with all your knowings for the following and after that following chapters or the future chapters. I beg of you to not leave me overthinking because as of now, I'm spiraling into uncertainty and I'm going insane, like insane insane. Thanks mwa love u.