Harris’s uncle, a twin (5)
I believe the answer is:
remus
This clue is a double definition.
'harris's uncle' is the first definition.
(Uncle Remus, Joel Chandler Harris character)
'a twin' is the second definition.
(legendary twins Romulus and Remus)
(Other definitions for remus that I've seen before include "Brother to fifty two across" , "Brother of 15" , "Twin founder of Rome" , "City founder - "uncle" storyteller" , "twin brother" .)