Aww, I'm sorry about your sister. I'm sure everyone waited to tell you because they just didn't want to disappoint you. It would have been nice if they told you sooner though!
My dad, cousin (and his gf) and my aunt waited until 1.5 weeks before the wedding to book their tickets and hotel. Luckily they didn't need passports for St Thomas, but it was still annoying that they waited soooo long to book because I wasn't even sure if they were going to end up going or not. Apparently some people are major procrastinators when it comes to travel plans... which I'll never understand, but I'm the super organized type. Anyway, I've heard so many stories about last minute bookers.
You are right, you need to just stop worrying about them. Focus on you and what you need to get done. They are adults, they can make their own plans and if they don't get all their stuff done on time, they have no one to blame but themselves.