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You and your sister have to arrange to defend the VCAT application otherwise it is very likely VCAT will grant your sister's application. Although you are not in Victoria, you could still be represented in the Tribunal and likely would be allowed to appear personally by telephone or videolink.
Whilst your sister may not be entitled to carer centrelink payments that does not mean she is not the guardian or joint gaurdian with you just because she doesn't live with your mother.
Your point that the tribunal should not appoint this sister who your mother had originally excluded is very much a strong point against her application, but unless you and/or your sister take action to oppose the application she may well get her way.
I would strongly suggest you and your sister (with whom you share the gaurdianship and POA) engage a lawyer to represent you as the odds of a positive outcome will be much better if you do so.
I trust the above assists.
Good luck and please rate my answer.