Have you considered the possiblity that she really doesn't want her family there? For all your angst, maybe this was a calculated move on her part. It doesn't sound like you have the happiest of clans. If she does want you there, then I would really give some careful thought to what it will say and mean to her if you don't go. If you truly can't afford it, that's one thing and it's a perfectly valid reason to skip it. But I'd try to look past your other reasons for not wanting to go to see what you might preserve and/or gain by going and what you might lose if you don't.
I am generally an anti-wedding person (I eloped) and I'm estranged from my own dysfunctional siblings, but even I can admit that sometimes you suck it up for the greater good when it comes to weddings and that they are legitimately something we should ideally celebrate with friends and family. It sounds like you do love your sister and her children and genuinely like her fiancee. I have to wonder whether you'll end up regretting it if you don't go (again, assuming you can afford it).
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher