One's twin deity (4)

I believe the answer is:

isis

I'm a little stuck... Click here to teach me more about this clue!

'deity' is the definition.
(I know that Isis is a Egyptian deity)

'one's twin' is the wordplay.
'one's' becomes 'i's' (Roman numeral).
'twin' becomes 'is' (I can't justify this - if you can you should give a lot more credence to this answer).
'is'+'is'='ISIS'

Can you help me to learn more?

(Other definitions for isis that I've seen before include "Goddess of the rainbow" , "where one might take a punt" , "River Thames at Oxford" , "Jihadists" , "water" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.