My guess is that some years after she died William came back to Ontario to live with his aging mother and died there and the grave of William Fraser in Fonhill Cemetery 1862-1940 is most likely him.
I posted the links earlier from findagrave for both mother and daughter Catherines', but if you look at the grave markers from canadianheadstones.com instead, you get the grave location. You can see that both Catherines and William are buried very close together, all in "Brown's South Extension - Row 28" and if you look at the id numbers in the link, extremely close together. (The id numbers would be taken from the order of the graves as the photographer went down the rows)
Just a bit more proof that the William is yours.
Catharine Upper, mother
http://www.canadianheadstones.com/on/view.php?id=629093[Brown's South Extension - Row 28]
Catherine Fraser, daughter
http://www.canadianheadstones.com/on/view.php?id=629097[Brown's South Extension - Row 28]
William Fraser
http://www.canadianheadstones.com/on/view.php?id=629099[Brown's South Extension - Row 28]
It might be worth your while asking the cemetery for the burial records on William just to see who took care of the burial arrangements. (Probably too high to hope it was brother John).
