I moved out when I was just turning 17. I had a full time job, in fact I earned more than my mum. This had prompted my dad to stop paying any maintenance at all, so in my eyes moving out actually benefited my mum and younger sibling. It certainly benefited me. I meant I could stop parenting my mum. Unfortunately this left my younger sibling to be parentified instead. I feel hugely sorry for this but wont accept the responsibility for our parent(s) terrible parenting.
Fast forward a couple of decades. My DS landed up at a PRU. He had a few friends there who were from or in the care system. I would go to the ends of the earth to stop any child from my family going into that system. It is not a system to "care" for children; it's a system to get rid of those unfortunate drains on society that happen to be children with inadequate parents. Children who have already experienced massive trauma in their families of origin. Children who are already normalized for abuse. Children who are absolutely ripe for grooming - either for sexual exploitation or radicalization.
My son was in the PRU less than 9 months and in that time he learned to pick locks, jimmy car doors, shoplift more or less anything, blag his way into someone's home... the list goes on. He learned a lot more from his "peers" than he did his teachers, that's for sure. We reached a crisis point when he was a "corner boy" (secondary supplier of drugs). His "boss" asked him to stab a rival dealer. Thank god, son declined.
My point is, the care system is a fucking phirana tank. Consigning your sister to that, you might as well have just left her for dead. There are a few, a very few, who rise above the background and become working class heroes. The vast majority end up leading lives of drudgery because they believe that nobody cared about them enough to get them out of that situation. Your sister knows that you have her back, and that is worth more than anything.
If your friends are judgey then I'd suggest they actually look and read reports of how bad the care system outcomes are. If they can't be bothered to look then they can fucking catch themselves on.
You are young yourself OP. Do you have someone older than you for support? A cousin, auntie, an internet friend? Or a nest of vipers such as MN!
You're doing a great thing for your sister. I see you.