Egyptian goddess, wife and sister 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)
'wife and sister of osiris' is the second definition.
(I've seen this in another clue)
(Other definitions for isis that I've seen before include "The upper Thames" , "Sister of Osiris" , "Si, si - an Egyptian goddess" , "To be, to be an Egyptian goodess" , "Divine female" .)