Query on birth of twins

Twins Mary Ann Holgate, twin and Susannah Mitchell - McAllister, twin (daughter of William Mitchell, Crew "Sirius" 1788 and Elizabeth ({unknown}) Fitzgerald - Mitchell - Wright - Wilkinson [Convict "Lady Penrhyn" 1788]) have conflicting sources of where and when they were born

http://www.environment.gov.au/shipwreck/public/wreck/wreck.do?key=7956 says they were on board the Sirius when it wrecked off Norfolk Island on 19/3/1790 (which would mean they were born either in Sydney or on the ship enroute)

http://www.fellowshipfirstfleeters.org.au/elizabeth_wilkinson.htm says they were born February 1791 on Norfolk Island

https://peopleaustralia.anu.edu.au/biography/wilkinson-elizabeth-27776 says they were born February 1791 on Norfolk Island

Can anyone add any sources for their birth?

https://peopleaustralia.anu.edu.au/biography/wilkinson-elizabeth-27776