Hi Julia,
Looking briefly at the family, it seems that Donald Rankin came from Morvern and his wife, Marjory McLachlan, born 1826 came from Ardgour. Donald's parents were Allan Rankin & Mary McLachlan, and from IGI records he had a brother, John, born 1808 Kingairloch / Strontian, Argyll.
From the 1841 census, the most likely parents for Marjory McLachlan seem to be Duncan & Mary McLachlan, they are at Clovulin, which is the right area for her Ardgour birth.
It would be a great help to know the names of Marjory's parents, do you have these from her 1883 death cert?
regards,
brigau