Just lost a long post! Vodka I think you made the mistake of making decisions without really thinking it through. You allowed your sister on the holiday without consulting SIL and then did the same with your ex.
I think you just wanted to keep everyone happy.
I think you are being unreasonable using the language of Suck it up for sis! But I think you got yourself in this pickle by not being more firm and decisive about what you wanted.
No idea what you should do.
But at the very least....
I think be prepared for things not to be easy with ex if you do go away with him.
Make sure your sis is not out of pocket.
Build bridges with your sis, don't bring in things from the past or use the language of 'suck it up'. Hopefully she will understand. The fact you were both willing to go on holiday together does suggest you have a closeness there, so hopefully you can work it out.
And I will agree with you about one thing, a trip overseas to Disney World is a holiday of a lifetime! I don't have much money at the moment so overseas travel does seem like a luxury. Next time something big like this comes up be decisive, and make sure you do enjoy this trip, don't let all this spoil it for you, that would be in nobody's best interest.
Bon Voyage. 