WOW

! Isn't it sad where peoples priorities are? Do whatever you feel is best. If i were you, I would do what the bride wanted, but what also works for you.
I would want to know a little bit of what was going on. just in case there is something you HAVE to tell her I would hate for her to be upset that you have been dealing with all of this. As for the friend with the priority "issue" she needs to wake up. This is not about what works best for her. The bride will appreciate all the hard work you and her sister do and will see who doesn't make the effort to go.
I'm so sorry you are dealing with this. I was married last June in Italy and went through so much stress about stupid stuff. I wish I had someone like you to help! You are a great friend!