You are right, of course. I think I may have interpreted the message incorrectly:
18 December 1858
THOMAS or MARGARET BURNS, from Seacombe, Cheshire, or any person to whom they may be
known, who sailed from Birkenhead in the ship Fortune, for Sydney, seven years ago, be kind enough to address to your brother, LAWRENCE KAVANAGH, Sydney Post Office.
Would it be that Margaret Burns is his sister and Thomas his b-in-law?
Did I see the Kavanagh's and Burns' living at the same residence on the UK 1841 census?
Attached is a PDF of Sarah - daughter of John and Margaret.
I don't know if I mentioned but NSWBDM online registers are restricted to births up to the current day and month to 1917, deaths to 1987 and marriages into the 60s I think.
As a result of the cut off dates you will see, in what I have found, that there can be a death without a birth. It gets very difficult to find folk and that is where death notices, cemetery indexes and headstone inscriptions, often come in handy.
Jamjar