Look at it this way: you are off the hook because you are not responsible for the invitations: your daughter is the one deciding who comes to her wedding. And she has made her choice. Wisely, from what it sounds!
Hopefully this will be something of a wake up call for your sister. The best way you can help is to offer to drive her to rehab or treatment and/or offer to go to an open AA meeting with her. I agree with the others that alanon is a great way to learn about setting boundaries, (what this wedding invitation is all about) and offering support.