Hi Katie, welcome to planDisney. Visiting the
Most Magical Place on Earth with loved ones is a special kind of magic. I hope you all get to enjoy some time together.
I'm a planner by nature, so I'd like to know what to expect ahead of my adventure. As you've booked through a third-party agent, you may like to alert them to your possible plans to see if they can work out a solution for your package. Please note that adding another adult could mean an additional charge for your room booking. You could also speak with one of the
Disney Experts before your trip, explaining your potential extra Guest. I advise linking your Disney Resort hotel reservation to My Disney Experience.
Once in Walt Disney World, you could add your sister via your
My Disney Experience account. Select 'Change Reservation' and add your sister from your Family & Friends list. You may also speak to a Cast Member at the front desk to assist. In my experience, Cast Members have always gone above and beyond to help Guests have the best experience possible.
I'd be happy to help if you have any other questions, Katie. Have a magical adventure! Louise.