Mom, to Auntie (3)
I believe the answer is:
sis
'mom to auntie' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sis that I've seen before include "Family member (informal)" , "Relative's" , "Girl" , "Secret Intelligence Service (abbr.)" , "Secret Intelligence Service (abbreviation)" .)