Have you looked at that 1881 census entry?
It is hard to see which surnames for the family are supposed to be Cowin and which are Corrin. Eleanor Maud aged 8 is entered as sister-in-law (of the Head, William, 25) - Daugh is crossed out. - Therefore Annie's sister.

The IGI has Eleanor Maud COWIN bapt 29 Sept 1872 Saint Mathews, Douglas, dau of Henry Cowin & Alice.
Henry Corrin (sic?) & Alice BROMLEY married 15 July 1858 Braddan.

William Henry was clearly given as son of William Corrin in the 1891 census along with the other children, all b.Douglas.

Sue