You could try..
"Are you trying to hurt me?"
this usually provokes the "what are you talking about?" or "huh?" kind of response.
Regardless of what they say in reply to that (this is a neat trick i learned from NTs) Reply with something like.
"Do you think your shirt/top/jumper feels too you exactly as it does everyone else?"
if they say yes
"No, because that's biologically impossible, peoples nerves are slightly different."
if they say no.. then..try
"Well being touched by someone doesn't ALWAYS feel the same to me as it does to you."
or whatevers 'true'.. often it needs a dose of shock, followed by logic (or near enough) followed by truth.
good luck
Also do some research on a big name murderer or such like you know they'd know of, someone who SEEMED normal..and if they throw "normal" at you again, just point out, "X was normal, everyone that know him/her said he was...is normal such a good thing?"
of course, Logic often fails in the face of parents.
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Waiting for the 'scientists' to realise thinking a 240volt dc fridge not working in a home with 240 volt ac mains mean