Goddess with one little sister (4)
I believe the answer is:
isis
'goddess' is the definition.
(Egyptian god)
'with one little sister' is the wordplay.
'with' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'little sister' becomes 'sis' (synonyms. I am not sure about the 'little' bit.).
'i' next to 'sis' is 'ISIS'.
Can you help me to learn more?
(Other definitions for isis that I've seen before include "God" , "Mrs Osiris" , "Sister of Osiris" , "Egyptian goddess - Thames at Oxford" , "The upper Thames" .)