Hi Carter,
Let me be one of the first to say "Welcome Home!" to Disney Vacation Club. It sounds like you have a wonderful vacation planned - a cruise and then a trip to Walt Disney World with extended family!
Being able to purchase
Tables in Wonderland is a great perk for Disney Vacation Club members. This card will give you 20% off food and beverage, including alcohol, at many Walt Disney World dining locations. In order to receive the discount, the Tables in Wonderland card holder - who I am assuming will be you - must be present, must present a driver's licence or state ID, and must pay for the meal. When using a credit card, the name on the credit card must match the name on the Tables in Wonderland Card. The discount is good for up to ten people.
Here is what I suggest for your meal at Cinderella's Royal Table. The Table in Wonderland discount is technically taken at the time of dining, although I have heard that sometimes Cinderella's Royal Table will take the discount at time of booking. It will need to be your name on the credit card in order to receive the discount. If your sister has already made the reservation for your entire party, I would suggest that you or she call Disney Dining at 407-WDW-DINE (407-939-3463) and ask that the reservation be changed to your credit card under your name. If the change is not possible, you could cancel her reservation and quickly make another one in your name with your credit card. Since Cinderella's Royal Table will give you a full refund as long as you cancel at least 24 hours ahead of time, you have plenty of time to do this without your sister losing any money.
I hope that this information is helpful for you, Carter. Have a wonderful time planning your trip, and please do not hesitate to ask if you have any further questions.