It's fab, which makes little sibling sick (5)
I believe the answer is:
brill
'it's fab' is the definition.
(I've seen this before)
'little sibling sick' is the wordplay.
'little' means to remove the last letter (the word is not quite complete).
'sibling' becomes 'bro' (I've seen this before).
'sick' becomes 'ill' (similar in meaning).
'bro' with its last letter removed is 'br'.
'br'+'ill'='BRILL'
'which makes' is the link.
(Other definitions for brill that I've seen before include "Flatfish; excellent" , "Excellent (abbr) - fish" , "marvellous" , "swimmer" , "First-class (informal) - fish" .)