I respectfully disagree with northeastern area and metoohurt...
Well no, I agree that your sister is supporting you, and you don't want to loose her.
But, IMHO, our spouse is our #1 family. Parents and siblings come after them.
When I got engaged to my H, my parents were greatly opposed. I informed them that I would always love them, and never break contact with them. But I also told them that he was going to be my H, and I hoped they would not make me choose.
They didn't. I never regretted putting him first, and they soon came around and learned to adore him.
Now, I know your wife has betrayed you and your sister is "in your corner", but again, IMHO, if you are both serious about your R, you and your wife are both in your corner.
You can love and appreciate your sister's support, and continue your relationship with her, but she needs to know that this woman is your wife - warts and all - if that's what you have decided. And that you will always love her (sister), but you have decided to R, and she needs to at least respect it, even if she doesn't agree with it or like it.
I don't mean to be harsh. Your wife betrayed you. But you have decided to R, so you need to be all in.
Please take this as a grain of salt...just one woman's opinion.
ETA: I realized I didn't answer your question regarding the wedding.
I would explain what I said above, (only if you agree with it, of course) and ask for her support for your wife to attend the wedding with you, so that the two of you could present a united front. If she refuses, I would consult your W, and ask her if she understood that you want to support your sister.
If your W doesn't want even you to go, you have a tough decision to make.
If neither of them will budge, then you would have a very tough decision to make.
[This message edited by WhatsRight at 1:38 PM, March 22nd (Thursday)]