Hi,
I see that Margaret and her family (parents, siblings) are in Winslow South in 1871 also.
So I assume, as you do that she would have married there and as John is shown as born Quebec and is same age as Margaret, I looked to see if there is a John Smith in South Winslow in 1871 and there is one of right age. To be sure of this John you will need to find some other record to confirm he is right fellow but seems possible.
1871 WINSLOW SOUTH, COMPTON, QUEBEC
SMITH
Malcolm, age 25, farmer, born Quebec - married January
Henrietta age 24, born Scotland - married January
Mary, age 55, widow, born Scotland
Norman, age 20, farmer, born Scotland
John, age 17, farmer, born Quebec
Murdoch, age 14, born Quebec
Donald, age 12, born Quebec
Donald, age 1/12, born Quebec - born March
1861 COMPTON, QUEBEC
SMITH
Donald, age 45, born Scotland
Mary, age 45, born Scotland
Malcolm, age 21, born Scotland
Ann, age 18, born Scotland
Isabella age 15, born Scotland
Effy, age 13, born Scotland
Norman, age 11, born Scotland
John, age 8, born Lower Canada
Murdoch, age 5, born Lower Canada
Donald, age 2, born Lower Canada