Try explaining that your nerves are overactive, and constantly send the wrong messages to your brain saying "pain".

To quote frogga:
"my joints don't work properly because the movement parts of my brain are over written with pain signals so my nerves don't send out the right messages."

Maybe your brother could understand that?

Or do you think he could understand some of what DayDreamer said?

How old is he? I have siblings and some understand. Reading this might help:
http://www.rsds.org/pdf/Recog_Undrstd_TreatRSD_530.pdf
(I am actually going to print some of these out for other people)

Also came across this:
http://www.rsdescape.com/RSD%20In%20Children.htm