Don't want to appear too negative here, but the first challenge you need to overcome is securing a passport for your fiancee's daughter.
In my opinion it's better not to spend money jusy yet on visa applications/preparations before the daughter's passport is issued.
Please look here at the webpage Department of Foreigh Affairs (DFA) - Passport Application Requirements
Look under the heading "For Minor Applicants (below 18 years old)"
Please pay special attention to the requirement under "If minor is illegitimate but acknowledged by father"
Does your Fiancee have the 'Affidavit of Acknowledgement and Consent to use the surname of father' ?