Hey there Diana, and welcome to the Disney Parks Moms Panel!
Sounds like a great family trip you have planned! There's nothing like enjoying Walt Disney World with family. I'm sure you'll have a wonderful time seeing it with your dear niece.
You don't say how old she is, so I'll assume your niece is a child. If her parents have already gotten her a Florida Resident Pass, she can use her admission card or MagicBand at the touchpoint to enter the Theme Parks and you're good to go. If she doesn't have a pass yet, an adult who is a Florida resident would have to show proof of residency at the time you buy her ticket (or pick it up if you've purchased it online).
If your niece is older and has a valid Florida driver's license, she can use it to prove residency herself. Otherwise, you would need her parent or guardian there to show residency and pick up her pass.
I hope this answers your question, Diana. Please stop by again if we can be of any more assistance.
Have a magical day!
Kathy K.