These are rare; most of the stories I read get another verdict. It's usually the morally correct side that gets to tell the story, but luckily for us, there are some exeptions where the narrator is blissfully unaware of it's implications in the story.
If you know me you know I'm not a big wedding connoisseur, I've only been to a few and there was never a 'rehearsal dinner', I don't even know what that means. Why would you rehearse a party? Anyway, if your sister needed you there, accepting money in exchange for a plane ticket she had bought, missing the rehearsal dinner, and keeping the money to yourself is kind of problematic.
It's obvious, she paid for the ticket for you to be at the rehearsal dinner and you won't be there, yes, the money you've accepted kinda belongs to her. Also, the sister could've been smarter and not tell her about it, right? She could've said the flight was overbooked and she couldn't make it, but she had to be petty and tell her sister she seized the opportunity to make some money for her using something she hadn't paid for in return. Weird.
I guess loose lips sink ships. Luckily they don't crash airplanes. I wonder if people are just ranting online when they tell these stories or they are honestly willing to change their minds if the verdict they get is negative. I don't believe them, I mostly think they are seeking for reassurance.
My conclusion is that if you're going to be that cheap, you should be more astute and not call yourself out at least. Now she broke her sister's heart and won't be able to keep the $1,200.
I don't judge her, I need money too, but well…