Hello again - I realised you had two unanswered posts before your Stevenson emigration query.
Based on your later question around Canadian emigration, are you asking about another family who emigrated, and if so approximately when? The more background you can give the more we can help.
If James and George grew up in Scotland then their subsequent marriage records might give you more information on their parents, including their date of marriage. I did have a very quick look at the '41 census to see if there was an obvious candidate but whilst there are a fair few James Moffatts in Dumfriesshire I can see one William father, one George brother but in different families and none with the William / Mary / George exact combination
