Hello Ashley! Welcome back to the Mom´s Panel.
Congratulations on your awesome planning for your first Walt Disney World family vacation.
Advance Dining Reservations is one of my favorite Walt Disney World services and I always make the most of the 180 days window.
Therefore, I suggest making Advance Dining Reservations for your family and your brother family. If necessarily, you can contact Disney at (407) 939-3463 and change your reservations at a later time. Adding guests to a reservation may be more complicated than subtracting them.
Also, your brother can make his own Advance Dining Reservations and later try to link them together. If he is not able to make the trip, he can then cancel his reservations.
Remember you can always call Disney Dining, (407) 939-3463, and ask them for the best way to accommodate your needs.
It is important to know that Walt Disney World restaurants automatically add an 18% tip to groups of six or more guests.
Also,
Disney´s PhotoPass + might be a great service to add to your family vacation.
Happy to help you plan a magical Disney vacation.
Feel free to keep writing and asking us all your questions.
Miguel