This thread has kept me thoroughly entertained while waiting news about Berge and Retsos. Thanks to all involved.
I am firmly in the one in fifteen camp. However, to answer Sean Thornton in a different manner. Yes, we do have some new information if Liverpool are drawn first, but it doesn't make any difference to the answer.
If Liverpool are drawn first, there is a 2 in 15 chance of this scuppering a Blades vs Wendy tie after the second ball is drawn (as there is a 1 in 15 chance of either the Blades or Wendy being the second ball out). Therefore, there is a 13 in 15 chance that Liverpool being drawn first won't destroy the chance of a Sheffield derby elsewhere in the draw.
If that doesn't happen, there are now only 14 balls left in the bag (if you'll pardon the imagery). That means the odds of an all Sheffield draw becomes a 1 in 13 for the remaining games.
The probability of an all Sheffield derby after Liverpool is drawn first but before the second ball is drawn is dependent on both of these events: 13 in 15 multiplied by 1 in 13. This will give you 13 in 195 chances of the Derby coming up elsewhere in the draw... Which simplifies to a 1 in 15 probability.
I hope this helps. If not, I can safely say there is a 1 in a million chance of an all Sheffield derby in the next round given the sty will turn into an abattoir when City visit.