Egyptian goddess with one little sister (4)
I believe the answer is:
isis
'egyptian goddess' is the definition.
(I've seen this before)
'with one little sister' is the wordplay.
'with' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'little sister' becomes 'sis' (synonyms. I am not sure about the 'little' bit.).
'i' put next to 'sis' is 'ISIS'.
Can you help me to learn more?
(Other definitions for isis that I've seen before include "Oxford's second eight" , "where one might take a punt" , "Si, si - an Egyptian goddess" , "Egyptian goddess and jihadist group" , "God" .)