Sibling's girls (6)
I believe the answer is:
nieces
'sibling's girls' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nieces that I've seen before include "They're family" , "They're related" , "Relatives of 27dn" , "Young female relatives" , "Scene I set up for relatives" .)