hmmmm its certainly a hard one isnt it?

When does she turn 16? Dont they get their own benefit then instead of the parents getting FTB?
Can you ring centrelink annomously (sp) and ask for some advice and see what they suggest. Saying that your are concerned that your Mum wont let her stay if there is a chance she will lose her FTB for her, maybe they have a solution for situations like this.

Best of luck and well done on being such a fantastic caring older sister. She may not know it now but she really will appreciate you in a few years