I think adding all the Australian details first is a good thing.
There is a tree on Ancestry which may prove helpful. A number of trees on that site are very unreliable, but certainly the details fit what you say.
It says that Isabella (Elizabeth) McGhie (1879-1955) , daughetr of William McGhie and Catherine Campbell, marrtied William Taylor (1872 - 1945), son of John Taylor (1827 - 1896) and Ellen Harriet Brown (1833 - 1903). It also shows John Taylor's death entry where he names his parents as Robert Taylor and Jane Brown.
IF this is true, then there is a baptismal entry on Scotlands People for a John Taylor, s Robert Taylor and Jean Brown, for 20/2/1825 in Falkirk, Stirlingshire. The only sibling I saw after a brief look was Jean Taylor on 14/5/1820.
There is an 1841 census entry in Falkirk:
Robert Taylor, 51, Mer(chant) S(eaman)
Jean T, 40
Margaret T, 16
John T, 15
Isabella T, 5
Janet T, 5
Robert T, 3
I haven't tried to find the siblings' baptisms but the family seems to fit.
In 1851:
Jean Taylor, 52, b Laurieston
Isbell Taylor, 16, dau, b Laurieston
Jean Taylor, 88, (great) grandmother
Women in Scotland often kept their maiden names, so it appears that John has already left home and Jean/Jane is looking after her mother-in-law.
Possible marriage: 1817 Falkirk Robert Taylor and Jean Brown
Possible emigration:
Ship: James Foord
1849 from London
John Taylor, 23, blacksmith, b Stirlingshire
All this is very speculative, but it may give you something to go on.
I do have two of the brothers' wills, but neither mentions any of your relations.
Richard