I feel for you. I wish there was a simple solution to offer. DH and I frequently discuss the "wanting our DD to have special quality one on one time with grandparents" as well. It may be selfish, but it is what it is and nothing will change that. Each set gets to spend TONS of time with other grandkdis, but very little with ours, comparitively speaking. My parents are 3 1/2 hours away and DH's parents are 4 1/2 hours away, so we feel like we are constantly trying to beg, finagle, plan, etc. time with them. MIL babysits my SIL's youngest as well as "fills in" for SIL/BIL whenever they want to do something or go somewhere, which means that we feel like we have to put in a request for time with them. We almost always go to them to visit, while they rarely make the trip to our house. My parents are a completely different story. (See below for a too long explanation.)
She was upset because I invited my brother and his family there for 2-3 days of the trip. I only did that because they live in Florida, so it would be convenient, and we don't get to see them very often - my sister lives a 1/2 mile away from me, she can see me and the kids whenever she wants. The other reason is because my brother is more relaxed than my sister - he would go with the flow. My sister would try to be in front of all the planning.
OK, I can kinda see where your sis could get upset with this part of it. (Not that I necessarily disagree with you though as my family's situation would be similar as in I'd much rather do something with my brother than my sister.)Anyway, after reading this, I thought how about this possible solution - could you invite your sis and family for those particular days when your brother will be there too? Is it at the end of the trip and they could continue on with theirs after you guys went back home? Maybe your sister would like to get together with your brother and his family as well since she probably doesn't get to see them much either?
long personal story: we have our first family vacation planned too and my parents were thinking about coming along for part of it. i actually thought it would be nice because i think we would have a good time with them and they would get to spend some quality time with my kids and also some time together alone (they never do). i was looking up some flight and room info and then called her about the prices and options and she says, "well, of course, we have to take your sister(21) and your niece(1)". i didnt say anything but i was so upset about it for days. they live with my parents (so it would actually have been an opportunity for my parents to get away from them) and my sister is very needy and demanding and i just know it would ruin the trip. not to mention that her daughter is younger than my kids and we would have to struggle with her nap schedule and my kids schedule. and because of my sister's personality my parents would have to take care of my niece and take her back to the room, etc instead of spending time with my kids. i was just really hurt because i thought the whole reason why they wanted to go was to spend time with my kids. i knew it seemed really selfish, but it just didnt make sense why they would have to come. (she's not my only sister, by the way... i have an adult brother and sister also. my mom didnt feel the need to bring them along, so it wasnt that she wanted the whole family to be together.)
WOW! Someone else who's situation is just like mine! I think of my sis as the black sheep of the family - every family has one, right??? My sis (33yo) and her 3 kids (6, 5, 2) live with my parents. It's hard for them to get away because my mom helps out so much with the kids, even though she still works full time. They should've retired last year (they are 66 now), but didn't or rather couldn't because of expenses. Oh, well. I would love a trip to Disney with my parents, but just my parents - not my sis and her kids. I look at it as an opportunity for them to spend time with DD without my mom having to watch/take care of my sister's kids, as well as an opportunity for the two of them to get away by themselves - which doesn't get to happen too often, unfortunately. When they do all come to my house, I try to take care of the her kids as much as I can while still trying to visit with mom and dad so that my mom and dad can play with DD. Where's my sis in all this, sometimes she comes and she helps with her kids too, but not a heck of a whole lot. Oh, it's just all so complicated, isn't it??? I try not to be bitter, but I feel so bad for my parents who should be enjoying retired life, not raising 3 more kids and still supporting one of their kids too. Of course, DH likes to remind me that it's by their choice that they all live there. Which is true, but the alternative isn't pretty...
SharpMomOfTwo, I apologize for getting off track from your situation.
Hopefully after everyone's had time to think, things will settle back down. Maybe your sis will decide to do the islands instead. Maybe they'd rather do Universal instead with her kids being the age they are. Maybe even SeaWorld. Hey, there's an idea. The WHOLE family can do SeaWorld together one day. The rest of the time they are in Universal and you guys are in Disney. Your parents can do whatever... Good luck with it all.
Seriously, I would wait and see what your sister has to say. You may get lucky and she may decide to go to the Carribean. I'm sure she doesn't realize what she is doing, especially since your Mom seemed to welcome her. If she decides that she'd like to join you talk to her and tell her your feelings. This is if you guys are close enough to discuss it without opening a can of worms. Good luck; I know this is stressful.
It's not the OP fault that she has her stuff together and isn't living with Mom. The sister is, I feel, taking advantage of their Mom. As an adult, she should be living on her own, supporting herself and DS and not living at home with Mom. Because of this fact, Mom is getting way more time with the other grandchild, and all family time is worked around the sister's needs, not the OP's. 
