I wouldn’t call her a child. She can get married, have sex, drive a car, join the army and she can get a job if she wanted to.She’s 17 - she’s a child and even so no matter her age we all say/do things we don’t mean when emotional!
Im not condoning what she’s said (if she said it) but just advising it’s prob best to take it with a pinch of salt as could be something she said in the heat of the moment!
I do agree it could have been a thing she said In anger and if she’s not talking to her sister it may not have been recent.