I'd advise against using the small, local car rental companies; their fleets are typically much older than the majors and they can sock customers with many hidden charges. Some even use GPS tracking devices to keep tabs on where the car is and levy a $1 per mile fee if the car goes outside California.

Enterprise is usually the best among the major car rental agencies for under-25 renters. A quick check of enterprise.com shows that most of the Bay Area/Silicon Valley branches do charge anywhere from $10 to $25 per day for ages 21-25, but your sister should be able to rent from the location at 211 Industrial Street in San Francisco without paying an underage fee (why no fee at this location only, I have no idea). Their phone number is (415) 330-0270.

I'm under 25 myself and usually rent from National through a corporate account, but when National is unavailable or overpriced, I've used Enterprise in the Los Angeles and Chicago areas. Both times they've given me nicer cars than I'd get with National (once a Nissan Maxima and the other time a Nissan Murano SUV, as opposed to the boring and underperforming Olds Aleros and Chevy Cavaliers of other agencies). Enterprise in SF may also be able to cut your sister a deal for a two-month rental. Just be sure she has adequate insurance, as credit cards typically don't cover rentals of more than 30 days.