Hi, Jennifer! The only way for your sister to include the Disney Dining Plan would be if she stays at a Disney Resort and there were tickets included in her vacation package.
Since she already has tickets, if she still wants to add the Dining Plan, what she can do is add on a one day ticket for each person in her travel party. As long as there is a one day ticket, the Dining Plan can be added. She can then use her tickets that she was given as a gift for the remainder of her vacation days.
You can actually click
HERE for the Vacation Booking Guide. When you get to the ticket type, just select a one day ticket. You will then be allowed to add on the Dining Plan. Try this a few times, booking with and without the ticket/Dining Plan add-ons to see what will fit the budget best.
If she is still having difficulty, call (407)W-DISNEY to speak to a reservationist who can help plan the perfect vacation.
As you help your sister go through the planning process, let us know if you have any more questions!