You probably already have the following info but it helps to see birthyears/places etc for background purposes
I know they married in 1868
Think you mean 1858
AMELIA MACKIE Spouse: ALEXANDER FYFE Marriage: 29 JAN 1858 Old Machar, Aberdeen
GEORGE SMITH Birth: 22 AUG 1838 Christening: 13 SEP 1838 Insch, Aberdeen, Father: WILLIAM SMITH Mother: AMELIA MACKIE
This means Amelia married 20yrs after George was born
1841 census for Insch - all b Aberdeen
William MacKie 76
Jane Cruickshank 71
Amelia MacKie 25
George Smith 2
Insch ED 4 Page 1 Line 1430
1851 for Aberdeen - both b Insch
Amelia MacKie 32 Teacher Of Reading Writing & Dressmaker
George Smith 12 Tailors apprentice
Aberdeen South ED 2 Page 2 Line 28
And that's where the trail ends for Amelia. I am assuming you have checked Scotlands People for a death for her
Marriages June qtr 1868
Esther Howard to George Smith Mackie Reigate 2a 215
FRANK HOWARD MACKIE Birth: 12 SEP 1874 Saint Nicholas, Aberdeen
Father: GEORGE MACKIE Mother: ESTHER HOWARD