Dont let this stress you out or ruin your wedding. This is yours and your fiance's day. You're sister in law had pleny of time to save money and book this trip. If she is not prepared to do so, then thats on her..not you. As much as Im sure you would love her to be there its not your fault that she waited last minute to book her flights. As for suprise guests, im sure your WC will be able to accomodate you. From what I've been reading on this site, its seems like no matter where you go, the WC are always trying to make this the best day for you. Good luck with everything and let us know how it goes.