Helen, you say John wasn't in the 1871 IOM census, but you need to look at these records more closely, and look at siblings as well. In 1861 the family is listed as:

Samuel 48 M Head Agric'l. Labourer Lonan Onc 1 53 Onchan Village
Elizabeth 33 M Wife - Onchan Onc 1 53 Onchan Village
Isabella Jane 8 - Daughter Scholar Onchan Onc 1 53 Onchan Village
John Maxwell 7 - Son - Onchan Onc 1 53 Onchan Village
Thomas William 2 - Son - Onchan Onc 1 53 Onchan Village

Samuel Brew & Elizabeth Cubbon married 16 AUG 1851 Onchan.

This looks to me like the same family in 1871:

Lon 3 134 12 Dumbells Road Elizabeth Head W 49 - Isle of Man
Lon 3 134 12 Dumbells Road John Son U 17 Sick Isle of Man
Lon 3 134 12 Dumbells Road Thomas W Son - 12 - Isle of Man

Elizabeth a widow, so Samuel had died between 1861 and 1871.

Although the census place [Lonan] is different to the 1861 listing [Onchan], didn't Samuel Brew senior die in Lonan?

This could be John's younger brother Thomas in 1881:
Household:
John LENNOX Head M Male 62 Lorton, Cumberland, England Blacksmith
Ann LENNOX Wife M Female 61 Lorton, Cumberland, England
John LENNOX Son U Male 38 Lorton, Cumberland, England Blacksmith
Thomas BREW Son In Law M Male 22 Isle of Man, England Farm Servant ((Indoor)) (Ag Lab)
Mary BREW Daur M Female 19 Lorton, Cumberland, England
John BREW Gr Son Male 2 Lorton, Cumberland, England
Ann E. BREW Gr Daur Female 2 m Lorton, Cumberland, England
--------------------------------------------------------------------------------
Source Information:
Census Place Lorton, Cumberland, England
Family History Library Film 1342248
Public Records Office Reference RG11
Piece / Folio 5173 / 63
Page Number 12

Alternatively, as you have found Elizabeth Brew als Cubbon's death in Canada, perhaps she emigrated with both sons between 1871 & 1881. - Don't expect second names to always be used in records.

The above census info. comes from Brian Lawson via the Brew web site, and from Familysearch.

Sue