First of all, don't do anything until you know for certain that your sister won't be coming. If she decides not to come you have several options.
Option 1: You can call the dining line and modify your reservations. Make it absolutely clear to the CM that you don't want to cancel your res, just modify it and make certain the CM knows how to do this. This is especially important with CRT where many CMs will do the easiest thing which is cancel and re-book. However, there's a chance that during that time period someone else will come in and pick up the res you just canceled and nothing else will be available for you.
Option 2: You can go online and see which reservations you can make for just your family. If it's close in time to your current res you will be asked which one you want to cancel, but a that moment you'll be holding both reservations so it's not like the CRT in the above example.
Option 3: According to the Disney dining FAQs page, you will not be charged the no show penalty as long as at least some of your party shows up for the reservation, so you could just arrive at the restaurants and let the CM at the podium know you're now a smaller group. Obviously, for the restaurants without a cc guarantee, this won't be a problem. I don't know if the same policy applies for CRT or the pre-paid dinner shows, however.