I also believe the same thing but last time I commented about it so many people disagreed. Her young age makes her also very susceptible to believe the brainwashing.
I agree that she is a child but she was never brainwashed. She was raised with a promise to get rid of the other person. It wasn't sugarcoated to her. "Look, here is the real wife and her real daughter. You mom is a mistress. Want to enjoy this lifestyle? Ok let's kill them" and she agreed.
Don't get me wrong tho, she is a child and children make mistakes. I would love to see her redemption arc while the adults perish.
Your*
Catherine's annoying asf but she turned out this way because of her real mother brainwashing her, she did genuinely loved the fl before