Thanks for your reply.
Ancestry visas are for those persons with british ancestry who wish to work in the UK:-
On this basis it would not seem appropriate for you to make an application under this category. If you did so, obtained the visa and then did not work it would be viewed as deception by the UKBA and they would most probably curtail your visa. This in turn would then prejudice any future application you would choose to make.
If you are not to work but are to visit the UK regulaly for a number of months per year in order to visit your sister then you should apply for a multi-entry family visitor's visa. You can ask for this to be granted for up to 5 or 10 years. There is a space for you to select which you want on the form.
Generally, you should not spend more than 6 months in the UK under this visa in any 12 month period.
Here is the page relating to this application:-
You should complete the form and submit bank statements for the preceding 12 months showing the amount of money you have available to cover your expenses during this time. In addition you should also submit any other financial records showing assets which are available for use to cover your expenses (including Land Registry Offficial Copes here in the UK showing that you own the property).
You shoudl also submit evidence of continuing obligations in Australia to convince them that you remain intent on remaining cheifly resident in Aus. Eg. Land title deeds, evidence of employement (if you are).
The above will need to be expressed in a statutory declaration by each of you confriming the details of your intended visit and referring to the above documetation.
Your sister should ask a local immigration solicitor to draft a sponsorship declaration in support of your application stating that she shall cover the costs of any unforeseen expenses that may arise during your stay here.
I hope this helps, if so please kindly click accept so that I may be rewarded for my time. It will be gratefully received and you will be free to ask follow up questions.