Hello, Renea!
We've been in this same situation often, so I understand your complication very well!
I suggest that you book the dining reservations as if the other family is going with you. This way, you'll have the dining reservations that you want now...especially helpful if they do, in fact, opt to go with you. Should this other family decide not to go with you after all, it will be far easier for you to change your reservation to a smaller party than to a larger one later on. I have often secured a second, smaller reservation (either online or with the telephone help of Disney Dining) for a night when I had thought we'd be joined by another family; we simply cancel the original reservation once I've made the second one, and have received a confirmation number.
Just be sure to consider those dining reservations which require a credit card hold to reserve them; be sure to cancel those which you do not intend to use well in advance, or you'll be subject to a cancellation fee. This Cancellation Policy can be found on Walt Disney World's site, on each dining location to which this applies. Check out this
link to Cinderella's Happily Ever After Dinner at 1900 Park Fare for an example of the Cancellation Policy.
Enjoy planning your upcoming visit to Walt Disney World! Be sure to send more questions as you think of them! I'd love to help!