Alfred died first, on 29 August 1848, buried on 31 August.
David died on 10 October 1848 and was buried the same day.
I could not find them in the list of burials in the old Sandhills Cemetery at the corner of Elizabeth and Devonshire Streets, Sydney, reclaimed to make way for Central Railway Station.
Today I looked at the list of burials in Sydney's first cemetery (the one that preceded Sandhills), the Old Sydney Burial Ground at the corner of George and Druitt Streets, Sydney, reclaimed in 1869 to make way for the Sydney Town Hall.
They were not there either.
Then I discovered that this burial ground had only been in use from 1792-1820.
It was too early, closing well before the boys were born.
The two Mawbey males died in the year after the birth of their sister, Ann Jane, and before that of their younger brother, John Thomas.
To view the list of many of the earliest burials in the colony of New South Wales, click on OLD SYDNEY BURIAL GROUND.
I have concluded that the two Mawbey boys must have been buried in the Devonshire Street Cemetery, and that their records have been lost.
If this is the case, they were buried in the same place as the mysterious Sarah Barckley Mawbey who was interred in April of the same year, 1848.