I know the thread's a bit old, but Isabella is shown on the 1851 census with the family and were neighbours of Margaret Fyffe/Fyfe an unmarried servant who lived along the street. I come to the conclusion that Isabella must have died sometime between 1851 and 1854, as Margaret Fyffe had an illegitimate son, John Powrie b. circa 1854 who I assume was the fathered by John Powrie the widower of Isobella. After his 'fling' with Margaret Fyffe, John Powrie Snr. fisher/weaver went on to marry Elizabeth/Isabella Hepburn in September 1857 and went on to have another 4 children. And as a matter of interest, Elizabeth/Isabella Hepburn's grandmother was another Powrie (Jean) who married Robert Webster in Errol in 1789.