Have I got this right?

Mary was 2 in 1841 - therefore born 1838/9
She arrived in Australia late 1850s - therefore max age 20
In the 1881 will there's no direct mention of daughter Mary - often means dead
If your Mary was Mary Handley, wife of Thomas (m. 31 Oct 1860 Onchan IGI ext) - dead? or gone to Australia leaving husband & two children
Handley children under 21 - therefore born after 1860
From this the date of arrival in Australia might need looking at again.