Nephew's sister (5)
I believe the answer is:
niece
'nephew's sister' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for niece that I've seen before include "Brother or sister's daughter" , "Preparation for curdling milk" , "Family relation" , "A sibling's daughter" , "[PRIEST'S] daughter?" .)