While most point of sale systems for Car Rental companies - Rental Management Software (RMS) - have the capability to charge for these daily extras for portions of the rental, these type of incrementals are usually added for the entirety of the rental. However, you should still ask the rental agent and/or manager when you rent if this is possible. If your sister is not there at the time of pick up, you will also most likely have to return to the rental office to have her added to the contract.