I think you are hiding behind this family obligation stuff.
Obligation or no, you decide what you are going to do. If you hate the choices you have been making, it's on you - not family.
The family obligation stuff I know about. I'd advise for you to throw that crap idea out. Do it if you want to - that's it.
I think once you give yourself permission to say "F this! F her! She's a brat, and a jerk, and I will not go to her wedding or do anything for her! What about me? It's always her her her!" ahhhh it will be so cathartic to you, and as those feelings start to get out into the open and cleared from inside you, the real deeper stuff will be able to surface.
It's not necessary to say it her, unless you really need to - but if you need to have it out with her, I'd do it NOW - time it waaay before the girl's wedding date. Give her time to work it through too. And in case it needs to be said, bring yourself to her after all the F yous are out of your system and you are speaking with a sense of control.
Screw obligation. Do what is right for you. Do it with others in mind, and no need to run over someone else in the process, but do it.
Not being in that wedding party is not a good idea - for you. People don't get this upset and wild unless they care, somehow somewhere. I'd wager to bet you care about her - even if this is someone you may never see eye to eye with nor be close to.
She's your sister.
I get the impression family is important to you.
My advice isn't exactly logical or noble, I guess, but hey, carrying around this garbage inside you has gone on long enough. Why not deal with it now...and go on to be in that wedding knowing you are there cause you want to (whether you like the man or not won't be the issue at that point).
You could turn a real page here, no matter what sis decides.