Biographical details held by the Society of Australian Genealogists, Sydney state surname as 'Jenkin', but I have seen emigration notes with surname as Jenkins for both Francis and Susannah. However 'Jenkin' is the more common surname spelling in Cornish records[1].
The thing to note is that his Son-in-law's Father is also named Francis Jenkin (or Jenken) and there may be confusion, and old documentation can have them both with the same last name.
ALSO could this be where Francis' two daughters (Susannah and Elizabeth) Married his Nephews (Francis) two sons ( George and Samuel) ???
England Births and Christenings, 1538-1975," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:J91G-PYY : 30 December 2014), Francis Jenkins, 11 Jan 1824; citing Gwennap, Cornwall, England, reference p 28; FHL microfilm 1,595,598.
Australia, Death Index, 1787-1985 Name: Francis Jenkins Death Date: 1885 Death Place: New South Wales Father's name: John Registration Year: 1885 Registration Place: Ashfield, New South Wales Registration Number: 3428
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