You won't find them on the 1881 Census as the ship
Potosi they were on left the UK on the 26 March 1881.
Edward Mostyn THELWALL d. 13 April 1899 at
'Ravensworth' Mildura, Victoria.
'Ravensworth' was the name of the home/property he was living at.
Edith Lawrence was possibly the owner or housekeeper.
John Bulkeley THELWALL d. 26 March 1889 at Fitzroy, Victoria.
His occupation was a Bank Clerk.
You will find the names of his brothers & sisters are in the files of his estate.
You will find their Wills, Probate and Administration of their Estates which were filed in Victoria, Australia, they also have the names of family from the UK & other interesting information available free to download from here:
http://prov.vic.gov.au/index_search?searchid=54There are 3 files for Edward & 2 for John.
Edward is probably buried at Mildura Cemetery. They are very good with requests for details & even photos of the grave if there is a headstone.
email: cemeterytrust
[AT]mildura.vic.gov.au
John is most likely at Melbourne General Cemetery. Again they are very good with requests for details of burials.
email: mgc
[AT]smct.org.au