I am not sure where your sister parks her car but have her check the fuel level after she parks her car and then again when she gets back in it. Do this for a few days, unless you have driven the car for like 50miles at a time and just seen the gas level just drop.
See if there is a fluctuation in the level; this should eliminate leak or theft. Start with the easiest first, as this is free and something she can check herself.
I cant see the car running normal with that terrible of mpg, something major has to be wrong. Can someone with an 08+ verify the fuel filter is in the same location as the 00-07's? If it is, its a job that you can do yourself. The filter is pretty inexpensive, vs a shop charging $100 for the job.