re that handwritten list that Cando kindly attached at reply #31.
To me, that's a snip of the information recorded on the birth registration as noted by Cando as a list of John's siblings. So, logically it will NOT have the details for John's YOUNGER sibling, and as it is the document for John's own birth registration, his name will not appear on the list of his siblings. So there's two of those missing names explained. (John and his younger sibling).
And as Cando noted there were no twin LANDELLS births registered in 1876 in Victoria, so that's the explanation for the third name not appearing on that list.
So, from your list westernwombat, it seems likely that the informant for John's birth did NOT know of the earlier birth of twin boys (add, just a year or so earlier than John's birth, but knew of the birth of Adam James in 1876).
I wonder who was the informant for John's birth?
From Vic BDM online index :
Registration year 1878, BIRTH, Reg. no 14677
John Dallawal LANDELLS
Father's name James Douglass
Mother's maiden name Sarah (Singleton)
https://online.justice.vic.gov.au/bdm/indexsearch.dojAdam's bc in 1876 is reference 8239. Twins are often registered at same time as each other, and allocated consecutive registration numbers. The online index allows searches by number. The ref 8238 for births in 1876 is for the surname ELLIOT and 8240 is surname MCDONALD
JM