A diminutive girl, my sis (5)

I believe the answer is:

missy

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'a diminutive girl' is the definition.
'Missy' can be an answer for 'girl'. I'm not sure about the 'a diminutive' bit.

'my sis' is the wordplay.
'my sis' anagrams to 'MISSY'.
But, I'm not sure how this is indicated.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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