Hi Duckweed...

I agree... people moved about to take up work where there was a skill shortage.
I am thinking perhaps that this Charlotte nee Jacques who married nJohn Black may well have been a cousin to Sarah and Thomas. Not sure where William fits in... but that he married a Charlotte Black... i.e. a Charlotte Jacques married a Black and a John Black married a Jacques. Is possible. That would mean that the signatory on the marriage certificate was not Sarah's sister but sister -in-law- wife of William if he is the other brother. i.e. Robert, William, Charlotte and Thomas. If Sarah Jacques was the only girl in the family of boys- Charlotte may have been the only one to fit the role of matron of honour.
There is definitely a Charlotte Jacques married to a John Black.She died at Prt Lincoln 18/5/1902 aged 95 ( good innings) as I found the record for the Pt Lincoln cemetery. Did not find a Charlotte Jacques however in any of the data bases that being as the wife of William also no record of William either- so he might have moved to Victoria, NSW or even back to England. So have no confirmation as to whom he married.
I took another look at the ICI records to see if i could find any William Thomas and Sarah all with the same place of birth and same parents but the only ones I could find were from Nottingham - Will 22 Oct 1815 Bradmore, Thomas 1818, and Sarah 1808- which can't be right as Sarahs birth was much later ( from her death record she was born 1819).
So. back to the certificate... if Charlotte Jacques and Thomas are on the certificate they are connected.
Thomas in 1844 so Charlotte is not his wife.
If it reads Charlotte Jacques nee B...k it usually means she was a Black but is now a Jacques.
Thus there is also a Charlotte Jacques who married a John Black and who came from Port Stewart or Dublin Ireland in 185? ??
Then there is an alleged John a brother of Charlotte Jacques so perhaps this Charlotte was the sibling of John and William and cousins of our lot.
On an ancestry post appeared some info stating there was a letter sent from John to Charlotte when they were in Ireland. John and wife had a daughter Alicia. When John's wife died, Alicia did not get along with the step mother when he remarried ( Mary Smith) and so she came to live with her aunty Charlotte here in Australia.
So I am still thinking there are two Charlottes both related- a Charlotte Jacques with brother John who married John Black and a Charlotte Black who married a Jacques ( perhaps William as he is the only male Jacques I know of who hasn't got a wife! Robert is married to Ann Newman. Thomas is married to Sarah.
They may have resided in England and then moved to Ireland.
To confuse the issue further, going back to the mid to late 1600s there were Jacques living in Scotland and Ireland who fled France- Robert, Guillaume and Jean and a Jacob. Unfortunately there are a few gaps in between and haven't found any trees with info on their descendants. I took a look at the Jacques from Yorkshire that I found on a tree. They stem from a Christopher b 1792 Doncaster married Ann Ward.... had John Jacques 1672 married Mary King.... had Charles b 1742 and William b 1739 married ???? ..... had Ann 1760, Eliz 1763, Mary 1768 James 1770 and Robert b 1758 married Ann????.
Robert had William b 1790 m Martha Malkin.. residing 1851 and 1861 census. ( Sunderland St). They had Robert 1816, Elizabeth, William 1821, Thomas 1825 Ann 1832, Mary 1836, Fanny 1841. By this stage we are at the same time when they came here. Is not our Robert b 1809 ( confirmed from death records in SA) although could be connected like 4th cousins.
It was stated that John Jacques - Charlotte's brother was a gifted guilder and carver- 39 Temple Bar St Andrews Dublin 1850 and 1851 at 10 Aunger?? St. Charlotte was at Coleraine Northern Ireland.
I have looked fro Ireland censuses but can't seem to find any info!
If she is in Ireland in 1851 then they must have emigrated about 1855.