Hi Mandy
Have yu checked out the DIAC website? If not, it is here:
Department of Immigration and Citizenship
I'm not sure about the procedure for claiming Oz Citizenship. ptlabs is our resident expert on Oz Citizenship. You can e-mail or send him a private message him via the forum, and his website is here:
Ptlabs Consulting
He would be able to answer the Citizenship part of your query, for sure.
With regard to your father and step-mother, they may be eligible for Contributory Parent visas if they can satisfy the Balance of Family Test. I doubt if your father's heart by-pass op would stand in his way as long as he is now fit & well again.
Please see here:
Family - Visas & Immigration
Hope this helps to get you started.
Cheers
Gill