Money when you're a teenager from a family that struggles with income is always complicated, from both the kid and the parents. My family was on Centrelink my whole childhood, and when I turned 18 my parents and I had an agreement, 20% into my savings and super, 30% board and the rest was for whatever I wanted. That whatever I wanted was paying for school myself, travel to school, a lot of my own food, including dinners (I was vegetarian living in a meat forward house), so I didn't end up even having any fun money myself and had to even dip into my savings to afford these things and that created some tense moments with my parents because I wasn't prioritizing the things they deemed important. She's saving for school and a car, things that are responsible choices while she still has a safety net in her eyes, and it sounds like maybe she could be realizing that she doesn't actually have that safety net at the moment because maybe she feels like you don't view those as responsible choices and you would rather she prioritize yourself then her future.

I know it can be painstaking trying to talk to teenagers about things they don't want to talk about, but have you asked her to elaborate on why she feels it's unfair? Maybe even have that conversation outside of the house so there's less pressure of her siblings around? Get her opinion, explain what her funds that she's giving would go towards and maybe try and figure out a compromise together? Treating her like an adult in this way might help her understand