DF's parents are my problem as well. Well his dad just will go with the flow but his mom is terrible about it. She has TWO years notice and REFUSES to save any money for the trip and basically told us that on top of paying for the wedding she expects us to pay for her trip as well.
I feel your pain - I really, really do! DF's mom was the same way. When possible, I stayed clear of the rift and let DF handle that side of the family. All I can say is it started out pretty roughly, but over time she's softened up and has warmed up to the wedding a bit as details are being finalized.
I would say over 90% of our family and friends are excited about a Disney wedding. Those are the ones who see it as a dream event and a vacation at the same time. DF's mom is the 10% who see it as spending unnecessary money and a pain to travel so far.
We proceeded with our plans the way we wanted because it was our wedding. With or without full support, we forged ahead. (Fortunately, we knew most everyone else was on board). We were always planning and talking about the wedding with our family and friends, but I requested that we don't talk to DF's parents much about it unless they ask. I knew whatever we'd say, DF's mom would probably make us upset. We'd let them know just enough details so they knew we were marching ahead and had everyone else's support. At first it was for my own sanity, but then we saw a slow change in behaviour that we didn't expect.
When it came time to book airfare, DF's dad also booked a ticket for mom. We recommended booking a Value resort and listed all the reasons why guests should stay on site, but they went ahead and booked a cheaper hotel on I-Drive anyway. Regardless, once they did that they were set to travel.
Fast forward a few weeks later, DF and mom were chatting and actually had a regular conversation. She asked how our preparations were going and was genuinely interested in details. That was... unexpected. Mothers usually like to stick their noses in wedding preparations though and I think maybe she felt left out. I think it just took her some time (a) to get used to the idea of a destination wedding, (b) to get used to the costs involved and (c) to see us work so hard and (d) to hear positive reinforcements about what we're doing.
I'm sure to everyone she's met lately, she's probably blabbed about our evil intentions of flying to Disney to get married. Instead of expecting a "how dare they do that!" reaction, she's getting a bunch of "how exciting!" reactions. After hearing that over and over, it's validating that this is more than an okay way to get married. So actually, her acquaintances might be your allies.
So now I have future in laws who didn't want to travel, but has made travel arrangements to be there. They're only staying two nights and won't visit a theme park. DF's mom has previously said she will not buy us a wedding gift due to their travel costs that we imposed on them. However, they will be there and that's more than what they were commiting to do a month ago. Maybe after visiting Orlando their attitudes will improve some more.
In the meantime, I feel for you because I went through every excruciating emotion over DF's mom. I think it just takes time.
Most of DF's family is in the panhandle, and my immediate family is in Tampa...extended family is mostly up north. 
), so we chose a date they could attend and then put subtle but direct pressure on them to book the trip. It's not the nicest thing to do, but in the end I believe they'll appreciate the trip, all the work we've put into it (we seem to be managing more details than if we just planned a local wedding) and Disney's ability to host special events.
It will all work out!
