My now 7 yo dd listed mk first and MGM last. I can't remember the order of Epcot and AK, but they're favorites, too.
With a one time trip for a 6yo girl, do the princess storybook breakfast if you can't get Cinderella.
My dd loves doing the kidcot and passport at epcot every trip. At Epcot, she also loves Test Track and Mission Space. We've eaten at Coral Reef and liked it. Liked the Land,
At AK, I'd recommend the safari, (of course!) Kali, if they'd like it, Lion King, Pocahantas (that's a must do!), Dinosaur if she's brave, Primeaval Whirl (if she's tall enough), and Flights of Wonder. Then there's also Tough to be a bug, etc....
IF she's brave, she's got to do tower of terror and RnR. My dd did both by 6. And of course the Beauty and the Beast show. Plus the character greetings often have princesses there.
There's too much to do in 4 days, I'll bet your sister comes back saying "we're going again!"