Brother or sister pops up twice (3)
I believe the answer is:
bis
'twice' is the definition.
(musical direction meaning to play a second time)
'brother or sister pops up' is the wordplay.
'brother or sister' becomes 'sib' (I've seen this in another clue).
'pops up' is a reversal indicator (in a down clue, letters go up).
'sib' backwards is 'BIS'.
(Other definitions for bis that I've seen before include "second time" , "To be repeated (music)" .)