Since it seems you do not have a lurabar disc problem, or something else in the spine that is causing the problem, you should be able to address the rotation through physical therapy and someone who specializes in body work. There may be an underlying cause for the rotation, such as foot pronation, one leg being longer (shorter?) than the other, etc. There is probably some sort of mechanical reason for the rotation. If you can find out what is causing it, you can correct it and then the piriformis should be able to relax and the sciatic nerve will no longer be irritated.
There are a variety of exercises you can do at home to strengthen the muscles and ligaments that will help hold the pelvic in the proper position. A short course from a PT or someone who really understand body mechanics could teach you the proper positions, etc.
From your description, it sounRAB like the Si joints are taking the brunt of the muscle irabalance....