Hey there, Suzann! Welcome back to the Disney Parks Moms Panel! It sounds like you have a wonderful family trip planned. I'm sure your nieces will have an amazing time - and you'll definitely get kudos for being the "cool aunt."
So, here are the basics of
Disney's Florida Resident Ticket program. The tickets can be purchased online, but you'll need
proof of Florida residency in order to pick them up and activate them. In the case of children, the adult Florida resident buying the ticket for the child must show proof of his or her residency before the child's ticket can be activated. In other words, your sister can buy the tickets for her daughters, but she'd need to be there in person when you arrive at the Park to activate them.
Here's one other option that might help. If your sister can get to Walt Disney World before your visit, she could buy and activate the tickets for your nieces at the Guest Relations Lobby at
Disney Springs, or at any of the Theme Parks. Then when you arrive, you can take the girls to Walt Disney World without her having to be there. I'd just have her confirm with Guest Relations which tickets she should buy based on the dates of your visit and watch any possible expiration dates.
I hope this helps you plan your family visit, Suzann. If you have any other questions, please be sure to stop by and see us again soon!
Kathy K.