Ahh, OP I feel your pain. Every family has the one that is the nurturing type, the one that opens her arms and doors to everyone and everybody. Only to be treated badly, judged harshly and shown little of the love and understanding she has shown everyone else. It frankly sucks and I am sorry for you.
The audacity of your sister, who has lived in your house, eaten your food, caused stress between you and your husband is apparently limitless. You are the only one who can and will advocate for you and your family. I am thrilled to see you doing that. Take that trip!!! Believe me, WDW is not a one time occurance for most people. You could go five times between now and the big family trip and still not see everything. If having this free trip in 2010 has strings attached to it to the tune of "you shouldn't be going without everyone else", than decline that free trip, it ain't worth it!
I would plainly tell your sister to bug off and that if she is jealous (because really that is what this is, its less to do with her daughter's heartbreak than her own dismay that you and your family are doing something she perceives she can't do.) too bad. You don't owe her a thing. She on the other hand owes you plenty. I know you love her, but she sounds like a spoiled brat. I love my sister and I could see her doing this same type thing. I have to love her and ignore her, if I didn't I would be forced to beat her senseless. Family!
And to the point of your niece. It does stink she might not get to go to WDW, although if you are willing and able to take her and your sister doesn't let her, than your hands are tied and you can't do a thing. But even if you couldn't take her and she didnt get to go, that is sad, but that is life! That is the problem with today's way of thinking. Too many of us feel entitled to all the things our friends and family have. Your niece will likely have things that come up for the rest of her life that point out that others have more than she does. Its called reality. Its not fair. Just like its not fair by some people's standards that she gets to live at her aunt's house while other little girls who have mommies and daddies that get divorced liven in shelters or under a bridge. Your niece is not your responsiblity. Again, I know how you feel. I have a nephew that was raised by my insane, selfish sister and he has had nowhere near the life experiences my kids have. I have done a lot for him but in the end, I haven't felt I could shoulder every expense that might come up that his mom can't afford. And guess what, he's 24 now, loves me dearly and spends his time with me telling me thanks for what I did, not giving me grief for what I didnt' . Your niece will hopefully grow up the same way, with a sense of gratitude and love, not the sense of ingratitude and entitlement her mom has. Only time will tell.
You'll work this out, because you love her. But take a stand now because if you give up this trip (and it sounds like you won't), than be prepared to give up something else the next time she throws a tantrum. It never ends unless you end it. And definitely start being petty and throwing her dirty clothes in a separate pile. And next time you talk to the bf, ask him when you can expect him and his U-Haul to pull up and bring her to his house since you are so mean.
Also tell your mom that if she can't see the beauty in her daughter's body, the body that has been shaped by the birth of her grandkids, that is a mighty sad statement of her character. And I feel for you on that one, because I have been told by more than one family member (on mine and dh's side both) that I have a non bathing suit worthy body. That hurts. But remember, if anyone feels good about themselves, they don't say things like that to others.
Hang in there, you aren't the one with the problem. They are. Enjoy your trip to WDW.