Yeah she does not have a mindset of an adult, she's still just a child in all tense and purposes. Imagine calling a orphan child a bitch, wtf is wrong with this woman.
The horrible environment she lived in is what shaped her to be the passive dumb adult a lot of readers keeps protesting her to be. She never teached to be brave, or to be the action type of person.
She's just a lonely kid/adult that cling to whatever she has, not ever knowing how to protect it.
I mean, the aunt is a glorified nanny. They see her even less as family as their adopted sister. She's just the help, and they intend to boot her as soon as she's not useful. Fake sister or no, if they weren't intending on keeping her she would have been gone long before.