Egyptian goddess, sister (and wife) of Osiris (4)
I believe the answer is:
isis
This clue is a double definition.
'egyptian goddess' is the first definition.
(I've seen this in another clue)
'sister and wife of osiris' is the second definition.
(I've seen this in another clue)
(Other definitions for isis that I've seen before include "Wife of Osiris" , "where one might take a punt" , "Revered figure" , "Goddess of the rainbow" , "Exists twice for a goddess" .)