J.S.
Okay... first and foremost you have got to STOP DOCTOR GOOGLING! ::slaps fingers off of the keyboard:
Did you read that? Did you listen? No?
::whaps nose with a newspaper:: NO! NO MORE GOOGLING SYMPTOMS!
Ahem. For real. What your sister needs to do instead of panicking is to call a local gastroenterologist and make an appointment. Someone reputable. That doctor will have all of the emergency room "stuff" sent over to look and will do his/her own examination and tests.
It could be anything right now, and I wouldn't panic. Panic means that no one has a clear head, right? Your sister needs to keep a clear head and so do you. Keeping a clear head means that you can think ahead to next steps. Until she can get in to see a gastro specialist, she should plan to take it easy on her digestive system. I'm sure the ER gave her a plan printout when she went home? If not, they should have.
If she's had vomiting, she can go on a low-residue diet or a clear liquid diet. That will help whatever is going on to rest her digestive system. She should also be measuring her pain in a journal. How often is she having bouts of pain, and when she does she needs to rate the bouts on a scale of 1-to-10 with 10 being "OMG I think I'm dying and need to be in the hospital 10 minutes ago." If she reaches an 8 on the pain scale, she needs to be at the ER. If she's vomiting and can't stop, ER. If pain is so bad she can't walk or move or breath or talk, ER.
If she has a smart phone have her install Manage My Pain Lite from Play Google. It's the free version and she can journal the pain and rate it and describe it. She can actually show it to the doctor/s and it has a great tool in it with graphs and everything. I use it for my Fibromyalgia flare ups and other symptoms.