If you look back at the 1881 and 1891 censuses there is a possibility that Sydney and William were brothers, the sons of Robert and Mary A Curran.
Well it is now looking more and more likely that they were brothers. The 1901 census you found for Sidney gives his age as 25 which puts his birth around 1876.
I can only find this entry which adds another element - an unusual middle name:
Births Jun 1876
CURRAN Sidney Wickens Swansea 11a 745
Now look at this WWI Memorial in the War Graves pages:
William Wickens Curran
Rank: Private
Service No: 69200
Date of Death: 09/08/1918
Age: 19
Regiment/Service: Royal Welsh Fusiliers 1st Bn.
Grave Reference: Plot 2. Row A. Grave 12.
Cemetery: GRANEZZA BRITISH CEMETERY
Additional Information:
Son of William John and Jane Curran, of 3, Garden Street, Swansea, South Wales.
So here we have the name 'Wickens' attached to Sidney and to the son of William and Jane.
Wouldn't have got there without your help, Groom. Cheers Malcolm