I heard that people usually get local anesthetic during an upper GI endoscopy. My sister is 15 and got a general anesthesia. Why?

Some sedation. Most people actually get some form of sedation for an endoscopy. The type used actually does induce light sleep so that the patient is not troubled by the endoscope. This is why some people feel that they had a general anesthetic. If you sister was not willing to have an iv, the anesthesiologist may have had to give her a mask anesthetic to get an IV in.
It depends . This is really more about what they were doing and what her condition is. Most upper endos are done with conscious sedation, some with deep sedation (with anesthesia), and some are done with general if more therapeutic maneuvers may be involved.
Usually twilight. She probably got an intravenous general no gas.