One familiar relative of Osiris (4)

I believe the answer is:

isis

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'osiris' is the definition.
(thesaurus)

'one familiar relative' is the wordplay.
'one' becomes 'i' (Roman numeral).
'familiar relative' becomes 'sis' (short for sister. I am not sure about the 'familiar' bit.).
'i'+'sis'='ISIS'

'of' is the link.

Can you help me to learn more?

(Other definitions for isis that I've seen before include "divinity" , "banker in Oxford" , "Sister of Osiris" , "Egyptian goddess - Thames at Oxford" , "Oxford's second crew" .)

I've seen this clue in the Evening Standard.

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