Was your sister age 25 or older at the time of the rental? And was the rental with one of the major companies, or a second-tier company?

For most standard rentals, enrollment in the major company's frequent renter programs (for example, Hertz' Gold Rewards program) means that the car is preassigned and that the insurance selections are predetermined at the time of the reservation - essentially no need to interact with a clerk trying to upsell insurance coverages.