I copied from another website what is available at the Aloha Isle in WDW. Here it is:
Dole Whip Soft Serve - pineapple, vanilla, or orange $3.49
Dole Whip Soft Serve Swirl - pineapple, vanilla, or orange $3.49
Pineapple Float - pineapple, vanilla, or orange Dole Whip with pineapple juice $4.49
Coke or Root Beet Float - with vanilla soft-serve ice cream $4.19
Fresh Cut Pineapple Spear $3.29
Assorted Chips $2.69
Coke, Diet Coke, Sprite, Root Beer, or Minute Maid Light Lemonade $2.59, $2.89
Smart Water $3.50
Bottled Water $2.50
Pineapple Juice $2.19
Orange Juice $2.39
Apple Juice $2.59
Hot Cocoa, Hot Tea $2.09
Coffee - regular or decaf $2.09
So, unfortunately for you, there is an orange flavor there. All may not be lost, however. You can ask to see the ingredients in the orange soft-serve -- this is usually a flavor base that a dairy product is added to. There's a chance that the orange flavor is not naturally derived.
Also, in my experience, places where the flavors don't change (unlike, say, my neighborhood frozen yogurt place which changes flavors daily), the pineapple soft-serve is always made in one machine, the vanilla in another, etc.
Finally, at Aloha Isle, you can see them handle the food and make your own judgement call. It's small counter service place where the soft-serve machines are in plain view. It is a very busy place, so in order to avoid any cross-contamination, your SIL may want to order things that come in cups vs. cones. A server may have a drip of orange soft serve on her hands, but they are touching the outside of the cup, not the inside, so she wouldn't ingest it unknowingly.
Have a safe trip!
DVCDawn