Although, as far as the "talented sister" is concerned, I think there's a reference later to her sister having her "design school friends" help her. But still, you're right. She would have demanded something from a famous designer, especially since she supposedly demanded Swarovski crystal.
And . . . she has a sister, but she "pretty much forced" her cousin, who obviously doesn't like her much, to be her maid of honor. Just cut it out. That part was added just so the cousin (the storyteller) could have a starring role in the story.
The more I think about it, I think this may be someone who stole her cousin's boyfriend and is trying to ease her guilt by making the cousin out to be so awful. . "I am so much better for him. She was simply awful." Oh, and the old "We tried to fight it, we really did, but our love was pure and true and bigger than both of us and just couldn't be denied." I mean, really, the whole thing about how they were in the courtyard and they both realized they still had feelings and she ran away because she is just so upright and good and pure and nice. Yeah, right. She jumped him.
And I thought the same thing about the parents. $50,000 is not a lot to remortgage a house in a "small, very rich town." Even if it's the smallest house in town.
Oh, oh, oh, and the guests. Now, think about it. They're all "very rich," don't forget. Yet, they put up with all this nonsense (costumes, being told what to wear, what to buy, etc.) and they rearranged their schedules on short notice and freed up probably 2 weeks, just because they were promised a "free cruise." Across the ATLANTIC, no less. Not the Bahamas, not the Caribbean, not Hawaii, not Greece. Nope. The North Atlantic. I doubt all of these "very rich" people would be that impressed. And then, come to find out, the day comes to board the ship, and their finding out they've either been put in "second class" or "steerage" depending on how she likes them. Can you just see a "very rich" couple boarding this ship, being told they're in "steerage" and saying, "Oh, sure! Can't wait!" Maybe we're sharing a bunk with Candy and Aaron Spelling!"