Hi Rob,
Regarding the 1915 marriage on the IGI, with Wm Henry's father being John and Ada Corkill's father being Robert, Brian Lawson's birth index has a possible sister to Wm Henry:
Lizzie Jane CORNISH, 8 Nov 1887, Ram, John CORNISH, Ann FARGHER
The alternative is COMISH:
William Henry COMISH, 12 Jan 1889, Dou, John COMISH, Emily KELLY
Siblings:
Alice Elizabet COMISH 27 Dec 1891 Dou John COMISH Emily KELLY
Emily Annie COMISH 24 May 1886 Dou John COMISH Emily KELLY
Florence May COMISH 18 Feb 1895 Dou John COMISH Emily KELLY
John Stanley COMISH 25 Jan 1885 Dou John COMISH Emily KELLY
John Richard COMISH 23 Jan 1899 Dou John COMISH Emily KELLY

The possible births for Ada seem to be:
Ada CORKILL, 14 Dec 1890, Mar, Robert CORKILL, Martha GELLING
Ada Blanche CORKILL, 7 Apr 1888, Malew, 100, Robert CORKILL, Mary COLLISTER

Sue