I was wondering if I could book a extra night (1 bedroom villa) for my sister on cash and then link it to the DVC reservation (1 bedroom villa) I have for her. This way they wouldn't have to change rooms.
If booked thru DVC, they can be linked.
If booked thru DRC, they can't be linked. You can have contact both DVC & DRC and have the other reservation noted.
I was wondering if I could book a extra night (1 bedroom villa) for my sister on cash and then link it to the DVC reservation (1 bedroom villa) I have for her. This way they wouldn't have to change rooms.
As Michael noted if the cash reservation was thru CRO (still better known than DRC) then it just has to be noted on each reservation. Your sister would want to verify at check in that they have her blocked to stay in the same room. We've done that before for my sister.