Honestly, the bolded part is the part that blows me away! Lots of family members don't like her, to the point that they don't want to be on vacation with her, but she has no idea??? That seems REALLY odd to me. Has anyone talked to her about the behavior they find unacceptable? Or does everyone just complain behind her back to you or whoever else? The whole thing seems crazy to me.
In my family, the nuts KNOW they are the nuts and are okay with it!

I would start by having a talk with SIL and gently reminding her that XYZ (since you haven't said why people don't like her) would cause a disruption to the trip and could she either refrain from doing so, or would she feel more comfortable in another room? Quite frankly, I think it's odd that you invited her knowing so many people can't stand her. If you didn't know it, I apologize...but you said the concerns are valid so I figure you must have known SOMEONE would be unhappy with her presence, and for good reason.
Like I said, in my family, the crazies are talked about openly and we try to work out solutions that make everyone as happy as possible. And if it were only a mild dislike or annoyance, we'd certainly suck it up for a trip to Disney World! Maybe that's just weird.

Good luck!!
ETA: I originally drafted this response before you posted the reasons no one likes her. Again, all I can say is that you knew all this before you invited her, so now you can't back out. If others don't like it, they are free to decline and stay home. Jeez...putting you in the middle like that...selfish people! You've been very kind and generous. Tell them they are free to pay their own way 100% in order to have their "ideal" WDW trip!