The 1901 census for Alberta and Saskatchewan Districts 202-206 has:-

Helen Cooil 59 born Ontario ?
Nettie Cooil? 13 Isle of Man, niece
John Cooil 31 Ontario
Lena Cooil 26 Ontario
Nellie Cooil ? 24 Isle of Man niece
Stewart Cooil 2 Ontario
Thomas Cooil 60 Isle of Man
Thomas Cooil 26 Ontario

It confirms more or less your original theory for Thomas 1841 but has changed the Macdonald name.

Len Faragher