Mom's sisters (5)
I believe the answer is:
aunts
'mom's sisters' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aunts that I've seen before include "Sally and others" , "Relatives get a peculiar US tan" , "Female relatives, usually older" , "Relatives get bad US tan" , "Relatives get an odd US tan" .)