I agree with Ryn, good undergarments will work wonders.
Go to DJs or Myer and get fitted properly - don't be shy of the old biddies they are wonderful. And they don't lie

You should have no problems walking into any of the stores you mentioned and looking great in a size 12.

Empire or A-line would be great, and I've seen shorter version on the tunics - more like tunic tops and Jacquie E (if its not too daggy)...how formal is the wedding?