Son is a close relative? (3)

I believe the answer is:

sis

'a close relative?' is the definition.
(short for sister)

'son is' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
's'+'is'='SIS'

(Other definitions for sis that I've seen before include "relative" , "Little sibling" , "Relative's" , "Secret Intelligence Service (abbreviation)" , "Family member (informal)" .)

I've seen this clue in The Sun.

Want a hint initially instead of a full solution? Install my app