OP you've now said you won't get the Police involved because it will cause more harm than good,your wrong there and I know that for a fact.
My parents had 3 children,I was the youngest,the oldest was a complete and utter cunt!and my parents defended him all the time and protected him constantly no matter what he did to them.He's stole from loads of us including my Mum near the end of her life and he was 54 years old then!
He's got broken marriages and children who's lives he's fucked up and now they've gone onto have children and they're all acting just like they're father always has,so my parents protecting him is going to lead to God knows how many more generations of that shit!
Back then it was very different as well,prison would have been awful for him but he did need it and my parents always regretted not calling them.
Now it's very very different and I say that as someone that worked with SS and who's husband is a Police Officer,get them involved and there's loads they can do now to help,there's charitys and organizations they can get your family involved with.
The Polices stance now is to get those that are more likely to be arrested in the future(your oldest Daughter is already close to that anyways) on side,so that they can reach out and stop these problems from happening rather than having to deal with them once the act is done.
You sound really depressed and I'm not surprised but you do have to deal with your oldest daughter you can not let her attack her siblings and there be no real punishment for it.What would have happened if your child had ended up in hospital OP?What if she does it again and goes further the next time?She needs to be stopped now before she gets even more violent.