No you cannot add your sister as dependent. Even parents here don't support their kids above 18. I have heard parents charging rent from their kids if they stay at their parents home.
Adding parent as dependent is only possible if you have a single parent. if you have both then they are dependent of each other, not yours. In case you have single parent then they have to pass family balance test. which means out of all their children more than 50% should be resident of Australia. e.g you are 4 siblings, you can only add them if 2 of you are resident.
Reason for all above is, why should AU tax payers take responsibility of people who never contributed to AU society and will never be able to contribute.
Moreover if you add someone as dependent and they don't meet criteria ( e.g Functional english, Medical, police clearance ), Visa application as whole will be rejected not just the person not meeting criteria, so don't get carried away.
