ICloud photostream pictures lost after upgrade?

I got the iPhone 5 today and when I was going through my pictures I noticed that I lost all the pictures that I took after I updated my previous iPhone 4S to iOS 6. My sister took my old iPhone so none of my information is left on there. On my old iPhone it showed that all my pictures were backed up to the Photostream through the iCloud. How can I get these pictures back?

Make sure your logged in to the account that you used to sync the photos to the icloud. If you have already done that then there's no hope for getting your photos back since they have been erased.