There are a few cardinal rules of genealogy which include :
5. ALWAYS have at least two separate sources of proof for each event...
6. REMEMBER that everything is only speculation until verified...
You remember Jessie Cameron? The younger sister of our Robert? Born 1839 and mentioned in the 1841 census as a 2 year old?
I didn't find any mention of a Jessie in the 1851 or 1861 census. Coupled with the inscription on Cameron Family gravestone in Urquhart Old Graveyard that reads "Jessie who died in infancy" I jumped to the conclusion that Jessie (born 1839) died young and didn't follow her line any further. I had what I thought were the two necessary separate sources of proof...
Confusion was the state I was living in when I read the 1871 and 1881 census for mum Janet who is noted as living with her granddaughter "Jessie Ann Crighton". Which daughter married a Crighton? I have Elspeth in Australia and Jane marrying a Horn and Jessie buried with her father... And our Camerons, steeped in tradition, probably weren't progressive enough to have the boys take a wife's name!
And therein lies the problem - names! I assumed that a. Jessie on the tombstone was born a Jessie and that b. the Jane Cameron I found who married a John Horn in 1845 was our Jane because, well, they were both living in Garmouth and our Jane, if the bride, was at the very marriageable age of 22 years!
And, of course, the universe has made an ass out of me because I now have evidence which suggests that little Jessie born 1839 and presumed dead actually lived long enough to marry and have a daughter!
The next reasonable conclusion then is that the child born Jane in 1823 is the child referred to as Jessie on the tombstone, (because Janet = Jane = Jessie and they seem to be used interchangeably) and Robert's older sister Jane therefore died in infancy.
Let me rewrite history then and note the following:
Jane Cameron born 1823 died in infancy
Jessie Cameron born 1839 married Philip Crighton in Garmouth on 14 December 1864 .
On 27 August 1866 they had a lawful daughter named Janet Ann Crighton (aka Jessie Ann) .
Sadly, Jessie died on 5 Aug 1867 when she was only 28 and Janet Ann was about 1. I don't have any record (yet) of what her dad was up to until his death in 1909 aged 69 while Janet Ann lived until 71, dying on 7 Apr 1938. 
 Thanks to U3A Lismore Inc
 1864 Marriage record for the parish of Urquhart for Jessie Cameron and Philip Crighton duly records the mother of the bride as Janet Cameron (Crammond) and the father as Alexander Cameron (Shipwright). And the bride is aged 25 which means she was born in about 1939.
 1866 Birth records in the parish Urquhart including Janet Ann Crighton. Father is Philip Crighton (Seaman) and mother Jessie Crighton (Cameron) married 14 December 1864. Informant of the birth is Janet Cameron (grandmother).
 Monumental Inscriptions: Urquhart Old Churchyard edited by Helen Mitchell and Bruce B Bishop p. 24, ref. 147