It's time to be this little girl's friend and mentor, but not her mom.
Perhaps some of the issues the siblings have together (you mentioned they are only beating on each other, and both parties seem at fault, not just the girl's sibling) is that they are uncertain of their futures, stressed, scared and overwhelmed with a need to "get it all out". Or maybe this is something they've learned from their former caregivers and both of them simply need a stable, loving environment to call home. Change might not happen overnight, but it could still happen.
However, I feel that even if you did take the family, your preference for first child will be felt by both, and cause additional problems, even if you try to hide it.
Your love for this girl can be shown by your willingness to let someone else love them both and perhaps even helping to find their forever home. I know that agencies and therapists and courts can be a bureaucratic mess, and not always right, but in this case, let the family stay a family, even if it's not with yours.