Obviously you need to say something to the bride. That's a pretty shitty way to find something like that out, and I'd agree with your ascertation that Ev & Aleathia had probably discussed this already.
That seems like the hurtful part (vs. Alethia just chosing her own wedding party, which you're right, she gets to do without a word from you.)
Sounds like your relationships with these people is pretty well deteriorated at this point, and I'd be tempted not to go, either. Just be careful you don't look like the Brat Who Threw A Fit When She Didn't Get To Be the Bridesmaid, b/c that's not what this is about BUT it's probably what people will be trying to paint you as. You can avoid that by telling the bride why you have a problem with the way she handled things, explaining that you don't feel comfortable attending anymore, and being sweet but distant from there on out. And send a cheap present.
Normally I'd say suck it up and deal with it -- she screwed up, definately, but depending on how big/traditional/how much input she's getting into this wedding, the bride might be really stressed out. Everybody has a story about the normally levelheaded friend who went off the deep end when planning her wedding.
I think your big decision here isn't about the wedding, though -- it's about if you want to continue to grow apart from these two couples, or if you want to try & mend fences for old times' sake. Sounds like you're kind of naturally growing apart as adults, and that's OK. But that makes way more of a case for not going to the wedding. Obviously
not going is going to create a rift you might not be able to fix later, so if you think you might be interested in becoming closer with these folks in the near future, go to the wedding anyway.