I'm sorry to hear that your sister is still having problems. Did your sister find out why she had blood in her stool the first time? That happened to me and it turned out to be two very small hemorrhoids.
I had my GB out 3 weeks ago. I've read alot of experiences from other people, and many of them have abdominal pains on the LEFT side, and it still winds up being the GB, so it could be the same in your sister's case. Does she get the back pain after eating fatty foods? ANd if so, do the abdominal pains happen at the same time? If so, it's quite possible that it could just be her GB. Let us know how her c-scope goes tomorrow. I'll keep my fingers crossed for her.