Hi Sue,
Thank you for your message.
Have you checked all documentation that you have for any mention of parents names? Documents such as marriage/death records sometimes list parents names.
Unfortunately, it may be difficult to prove the relationship here. Catholic records for Donacavey 1857 do not start until 1858:
http://www.irishtimes.com/ancestor/fuses/rcparishmaps/index.cfm?fuseaction=showidrecords&CityCounty=Tyrone&parish=Donacavey&churchid=302
However, in the19th century new Catholic parishes were created so starting dates can sometimes be misleading. Many parishes changed their geographical spread so you may find earlier records relating to your family in the records of adjoining parishes:
http://www.irishtimes.com/ancestor/browse/counties/rcmaps/tyronerc.htm
Most Catholic records are still held locally so it may be an idea to write to the local parish priest for possible assistance.
Most surviving Church of Ireland records are still held by the local clergy, although some are in the National Archives of Ireland and others are in the Representative Church Body Library in Dublin. Here are their websites: http://www.nationalarchives.ie/ and http://ireland.anglican.org/about/42
Lists of these surviving registers can also be found at the National Library of Ireland.
Alternatively you could try looking into the girls emigration. If the two girls emigrated together it is very likely that they are related. The Public Record Office of Victoria has good online databases of settlers at www.prov.vic.gov.au Otherwise, other records may be found in the Colonial Office Papers of the UK National Archives, class reference CO 201. This class contains a wide variety of records, including petitions for assisted passages, emigrants' lists, records of emigrants on board ship, petitions from settlers for financial assistance and much more.
Best wishes,
Genealogy Support
