Fast Pass + Dilemna

If you are buying your nephew's ticket at the window when you get to Disney, you will have to wait until that day to get his FP+ selections. If you have a smart phone with the My Disney Experience App, as soon as you buy his ticket, you can add his ticket to your MDE and make FP+ selections for him. But you will still have to wait until you buy the ticket. If you don't have a smart phone with the MDE app, then you have to visit the FP+ kiosks in the parks. There is no way to make FP+ selections for him until he has a ticket of his own. If you wait until the first day of his visit to buy his ticket, there is a very good chance that he may not get any of the same FP+ options that your family pre selected. The only way the whole family would get to have the same FP+ is to change everyone's FP+.

The only way to avoid him being left out, and your kids having to change their FP+ when they get to Disney, would be to purchase his ticket now over the phone or online and add his ticket number to your MDE. After you've done this, you can make FP+ selections for him too. You may have to change around the times a little of the FP+ that your kids already picked, but at least in this scenario all the kids involved would have the same FP+ times and selections before the vacation. If it was an adult, I would say go ahead and wait to buy the ticket until you get there. But since there are children involved, you will definitely have meltdowns from some, or all of them, concerning the FP+, if you wait until the day of to buy your nephew's ticket.

Yup, Annie about summed it up.

Depending on what you're FPing, if one or more adults will be a non-rider, you could put their band on the child so he can use their FP.

Otherwise, I would make a point to get a ticket ahead of time and link it to your MDE account. We recently traveled with my parents and we had no trouble with FP synchronization. We are on AP's and they were on a 5-day MYE ticket. We all stayed together and all had magic bands. DH did have trouble with his band linking to the account, but that was from an unrelated issue.

I agree with Annie