Hi Aussiegirl,
If you're in pain, how about a high stool you can sit/lean on during the ceremony? You can stand at the beginning, but once everyone's eyes are on the bride and groom you can cheat a little bit. I know standing in the same place is one of the worst things for me. I'd rather walk for an hour than stand for an hour. I'm sure no one would mind, and your sister is just happy to have you there.
Just remember that the important thing is doing the best that YOU can. And like Pea said, make sure you make some time for yourself to relax.
Have a great time at the wedding!
