That guy’s sister starts to make sibilant sound (4)
I believe the answer is:
hiss
'make sibilant sound' is the definition.
(I've seen this before)
'that guy's sister starts' is the wordplay.
'that guy' becomes 'his' (I am not sure about this - if you are sure you should believe this answer much more).
'starts' indicates taking the first letters.
The first letter of 'sister' is 's'.
'his'+'s'='HISS'
'to' is the link.
Can you help me to learn more?
(Other definitions for hiss that I've seen before include "Audio interference" , "Sibilant sound made through the teeth" , "S-s-snake's s-s-sound?" , "Make sibilant sound like snake" , "Urgently whisper" .)