Hello, Layshandria!
A sister trip to Walt Disney World, using Disney Vacation Club points in September sounds fantastic!
Because you'll be staying with your sister, using her DVC Membership, you can both add the Disney Dining Plan onto her reservation. Your sister, as the Member, will have to call Member Services to have the Dining Plan added for both of you, and you'll need to add this for each night of your stay. (For example, if there are two people in one room, staying two nights, then both need to add on the Dining Plan for both nights.)
All of your dining entitlements will be added to your Key to the World Cards, in one "pool." As you use your entitlements, they will be reduced from your total. Look at this
Disney Dining Plan PDF, which gives more details and examples of how to best use your plan.
For DVC Members, the current cost of the Disney Dining Plan is $51.54 per night, per Guest 10+; this entitles each Guest to one snack, one Quick-Service Meal and one Table-Service Meal per night of stay. None of these entitlements will expire until midnight of the day of your check out and gratuities are not included in this price. You'll also each receive a resort-refillable mug, which can be used at any fountain service location at your resort for the duration of your stay.
Your sister will have to ask for this to be added to your reservation at least 48 hours in advance of check-in, and will have to pay for it in full at that time.
If you are planning on eating at any of the park or resort restaurants, you should seriously think about adding on the Dining Plan. (We like to order the tastiest, most expensive thing on the menu, and don't feel guilty because it's included in the plan! Delicious. :-) )
I hope this helps! Please send additional questions as you think of them; I'd love to help!