Painful for Londoner's siblings who had skin scratched to retrieve DNA (8)
I believe the answer is:
blisters
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for blisters that I've seen before include "Swellings filled with air or fluid" , "Air bubbles in a coat of paint" , "Bubbles of water on the skin" .)