Sibling from South Island (3)
I believe the answer is:
sis
'sibling' is the definition.
(short for sister)
'south island' is the wordplay.
'south' becomes 's' (abbreviation).
'island' becomes 'is' (geographical abbreviation).
's'+'is'='SIS'
'from' acts as a link.
(Other definitions for sis that I've seen before include "Family member (informal)" , "Secret Intelligence Service (abbr.)" , "Secret Intelligence Service (abbreviation)" , "Girl" , "spies ..." .)