Hi Michael! Always good to hear from my fellow North Carolina residents!
If you booked your vacation with Disney directly, you can add an additional person to your reservation by calling 1-407-W-DISNEY no later than three full days before your trip. (If you booked your trip with a travel agent, you should contact your agent to find out if their policies for changing your reservation differ.) You don't mention how many people are in your travel party, but keep in mind that adding an extra adult to a room may cause additional charges and that not all rooms accommodate more than four people.
From a financial standpoint, it's better to add a person to a reservation than to remove one. If you booked a package, changes within 45 days of arrival incur a $50 modification fee, but the fee is typically waived if the changes cause the total cost of your package to increase, rather than decrease. Thus, adding a person usually incurs no fee, but taking one off within 45 days likely will.
Adding your niece to your dining reservations will be a little trickier. The restaurant reservation system is not set up to easily add another person to the dining party so you will likely have to cancel and re-book your dining reservations for the new party size. If you reserved any hard-to-get restaurants, I recommend that you don't cancel those reservations until you've made new ones. If you're unable to make a reservation for your larger party at a particular restaurant, keep your original reservation and when you arrive at the restaurant, just tell them you now have an extra person. In most cases, they will be able to accommodate you with no problem.
Hope this helps! Feel free to contact me again if you have additional questions!