Brother's or sister's daughter (5)
I believe the answer is:
niece
I believe this is a double definition.
'brother's' is the first definition.
(thesaurus)
'sister's daughter' is the second definition.
(I've seen this in another clue)
(Other definitions for niece that I've seen before include "A sibling's child" , "Family relation" , "Son's cousin" , "Brother or sister's daughter" , "[PRIEST'S] daughter?" .)