I don't think Fahlada is going to weak because NangFah is not weak. She is still the same girl, remember?
Everyone has weak points though - hers is trusting and loving her evil sister too much. I can't blame her though. Prae is all she got left after her parents drowned. Prae also behaves like her parent in controlling her now and Fah takes it as it's coming from sister love because she doesn't see the evilness in Prae yet.
Fah is going to fight HERSELF to not falling in love with Charles. She think she is doing it for the right reasons - because of her sister. It will be a battle between mind and heart for Fah.
Lookie - She might not realize it but she is already falling for Charles.....He makes her happy~