Hi, have you got any further with this or are you still in a rut with the other bridesmaids?
to make things easier you can ask them to book their own flights or accomodation. all you have to do is send them the details and a payment deadline and then its up to them, otherwise you end up out of pocket.
make sure you have a spreadsheet that you tick everyone off when they are booked and paid.
assign small tasks to the most reliable ones, for instance are you wearing matching sash's or t-shirts, get someone else on that, just send them all your research. make sure others are responsible for taking things over to spain for you. like games or funny props that others can take in their cases.
make sure the bride knows exactly how much the other bridesmaids are failing.
i've had the same situation myself. im thinking of starting a business called 'sort the bridesmaids out'! im sure plenty of people will pay me to take the pressure off! hahah
hope you're managing!