Sis's sib (3)
I believe the answer is:
bro
'sis's sib' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bro that I've seen before include "US form of address" , "Sis's sibling?" , "somewhere one feels close to" , "who may have a 28 [SIS]?" , "American greeting" .)