Hello, Meri! It sounds like you and your niece have a very special relationship, and I'm so excited that you have the opportunity to travel to Walt Disney World with her next month.
Elsa and Anna are two of my favorite Disney Characters, and I think it's really great that you and your niece were able to find coordinating costumes for Halloween. Unfortunately, adult Guests of the Walt Disney World Resort are not permitted to wear costumes, with the exception of Mickey's Not-So-Scary Halloween Party in the fall.
This shouldn't discourage you from showing your Frozen Disney Side, though. You are welcome to create a Frozen-inspired outfit to wear to the Park of your choice, so long as your outfit does not fully resemble a Disney Character or costume. In fact, there are many adult Guests that choose to dress like favorite Disney Characters. This style movement is referred to as "DisneyBounding," and you can see all kinds of really neat clothing options through a quick internet search.
Before you start packing for your vacation, make sure to read more about
Disney's guidelines for appropriate attire if you'd like to put together a Frozen-inspired outfit. I'm sure that you will look absolutely fabulous in whatever you come up with!
I hope that you and your niece have a wonderful visit to Walt Disney World, Meri. If you have any additional questions before your trip, please do not hesitate to contact me here at the Moms Panel. Happy planning!