This sounds like a fabulous celebration planned at Walt Disney World! Congratulations to your husband and his sister in their upcoming reunion!
I would certainly make at least one dining reservation at a table service restaurant where you can toast this great reunion and have conversation. You won't get caught up over 17 years in one evening but it will be a good start. Locations that have a bit more privacy include Yachtsman Steakhouse in the Yacht Club resort and Citricos in the Grand Floridian. Check to see if the wine room is free at Citricos as this is a room in itself you might be able to get and seats about 12 persons. My first recommendation for dining in a park when you want to have a relaxed and special evening is Bistro De Paris located on the second floor of the France pavilion in EPCOT.
To add to a special dining experience have a cake delivered to both of them as a surprise at the end of the meal. You can customize it by calling the cake hotline at 407-827-2253 and they will handle all the arrangements.
I would also consider having something special delivered to each of their rooms. The Dream Makers at the Disney florist can help you customize something that would be appropriate and special for them both. You can see many of the options by going on line to
www.disneyflorist.com and then call and speak to the cast member directly.
The ideas are endless: fireworks cruise, tours, customized "ears" or shirts. Disney will help make the magic come true in many fun ways depending on how far your budget is able to go. But whether it is a big or small celebration it will truly be important and magical at Disney!