There are 2 options:
1) keep the points seperate, but 1 person book days 1-3 and the 2nd person book days 4-7 (or however days work out). Give the reservation # to the 2nd person so member services can link the 2 reservations together(so you don't switch rooms)
2) points transfer.
Arrange a 3-way call with MS and have your year's points transferred to your sister (or her to you). Then 1 person gets the points to make a continious reservation.
You only get 1 transfer in or out per use year. (So if your plan dalls theough, your sister still has the points. Borrowed points cannot be transferred, but I think banked points can be transferred (or is it transferred points can be banked?)