i say you need to arrange a trip to go and see them.by being there and talking in person everything will get sorted out there and then.be prepared to be yelled at even though you don,t deserve it and they will be in the wrong but they won,t change.do it asap so you don,t worry about it for too long and the longer you leave it the harder it will become.explain that you are paying for the wedding and it would mean the world to you if they were there and leave it with them.don,t kiss their a**e its your day and you are doing what you want to do.
good luck keep us in touch