Osiris's one little sister? (4)

I believe the answer is:

isis

'osiris's' is the definition.
(thesaurus)

'one little sister?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'little sister?' becomes 'sis' (little sister is a kind of sis).
'i'+'sis'='ISIS'

(Other definitions for isis that I've seen before include "The upper Thames" , "Old Egyptian fertility goddess" , "divinity" , "The goddess lives twice" , "Oxford's second crew" .)

I've seen this clue in the Evening Standard.

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