Relative in Secret Intelligence Service (3)

I believe the answer is:

sis

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This could be a double definition.

'relative in secret' is the first definition.
'sis' can be an answer for 'relative' (short for sister). I'm unsure of the 'in secret' bit.

'intelligence service' is the second definition.
(I know that SIS is a intelligence service)

Can you help me to learn more?

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

I've seen this clue in The Sun.

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