I like Champs idea. I would keep her in the wedding but don't give her stuff to do. Friends start coming out of the woodwork offering to help so use other people if you need to. I think giving her a goal to get better for is key.
As an aside, my mother was first diagnosed with breast cancer when she was 31 and she had three of us, she battled the cancer and won until recently but its possible and keep hopefull. The one thing my mom talks about is that without the pressure of feeling better for us she might not have made it through so give that goal to your sister of the wedding and hope for the best.