Hi Angelique! Welcome to the Disney Parks Moms Panel!
In the past, it has been my experience that when picking up tickets at will call, they ask for photo identification as well as the credit card that was used to place the order. The cast member may be able to give your sister her ticket since it is listed in her name if she shows up with photo identification however I am not 100% sure of their ability to do this. If she does have to pick up her park ticket, tickets can be picked up at any guest service window at any of the four Disney theme parks and also at the
Downtown Disney Area. Also if your sister has a magic band, you should be able to link her ticket to her My Disney Experience; therefore bypassing the need to pick up the ticket at will call.
My best advice to you would be to contact the New Ticket and Special Events phone line listed on the
Walt Disney World Phone Directory portion of the website, and ask them exactly how they recommend your sister should pick up her park ticket since she is arriving before you. This way she should be able to avoid any surprises once she arrives!
I hope you have a pixie dusted vacation with your sister and family!
~Heather~