I came across this funny little glitch.
Face Sibbi Blackbriar and then wait for a few hours, he shall magically jump prison cell after the wait is over. Pick the lock of the cell his is now in and then you can shout, stab and fire arrows in to him or pick his pockets. Choise is yours people!
This happend each and every tiem that I waited, and if I picked the lock on the cell next to his; sometimes he popped out of his own cell, walked along to the cell next to his and strolled on in.
I did video it, but for some reason I can't post links in this post as it tells me I can't!