Ten grand is a decent amount for a reliable used car. Try to find one that STILL has some warranty left on it. That way, if any issues arise subsequent to purchase, you are still covered under warranty. That is important. Too often, used cars are sold in "as is" condition and I would NEVER depend on what the salesperson has to say about the condition of the the vehicle. In all likelihood, they paid very little for it on a trade-in. Car dealers make more money on used cars than new cars! So, if possible,  insist on a car that still has some warranty left on it and even see if you can extend the warranty. An investment of an extra $200-$300 for an extended warranty is WELL WORTH it. A single problem with transmission or air conditioning or other issues could cost well over $1000 or more. You know how salespeople can be. Once they sell the car, they don't know you anymore! So protect your investment!  And SHOP AROUND! Don't let your sister be too quick about parting with her hard earned savings. With a car, it's not just looks and appearance. It's mechanical condition! You want a RELIABLE automobile that will serve you well.
 
As far as what brand she wants, that's up to her. In view of current and expected future GASOLINE costs, look for fuel efficiency. Fuel-efficient used cars may not be easy to find because everybody wants them. Don't let them push a gas hog on you. Once you decide what you want, call your insurance agent BEFORE you sign a sales contract and find out EXACTLY what it's going to cost to insure the vehicle per your state laws. If any portion of the vehicle has to be financed, the lender will have insurance requirements as well to protect their loan. 
 
Toyota and Honda and good cars. There are also some good American cars with excellent warranties. Generally, foreign cars are expensive to repair so weigh all that into your decision. For example, I love Toyota and have owned a couple of them. Are they "cheap to keep" as their marketing says? NO! They are not. Repairs can be way more expensive than American cars. 
 
Finally, do your homework on the internet BEFORE you even go to a dealer. Visit auto websites and read reviews and repair histories on vehicles. Also go to 
www.kbb.com (Kelly Blue Book) for REAL pricing and an analysis of various cars. Also, as someone else mentioned, 
www.cars.com    Don't take the dealers word for anything, no matter where you go. Get it all in writing. Fight back and don't get ripped off! When buying a used car, you need to be "armed" with as much knowledge as possible. The last thing you want is to dump hard earned money into somebody else's "lemon".   
 
Hope this helps.