I am afraid that it's impossible to answer this without lots more information. It is true that if type-1 diabetics stop taking insulin, they will generally become ill very quickly. However, if the diabetes is very well controlled, sometimes it can go into remission - in which case it might take a while for the symptoms to reappear. Mind you, this has usually been seen in children, and although you don't say how old your sister is I presume that she isn't a child. Therefore, assuming that she has been off of insulin for a while, now, it does sound as though she has been misdiagnosed. This happens quite a lot - although type-1 and type-2 diabetes are very different conditions, they can can superficially look very similar. The most reliable way of telling the difference is what is called a "C-Peptide" test (if your sister hasn't had one of these, she should probably ask for one). It is also possible that she has some other form of diabetes - there are quite a number other than the usual type 1 and type 2 (although they are quite rare). Sometimes these other forms of diabetes are dubbed "type 1.5".
Why did the doctor put your sister on insulin six years ago? Well, presumably he or she thought that it was the right thing to do at the time. If her BG was badly out of control, then insulin would probably have been the quickest way to get it stable at safe levels. If a C-Peptide test wasn't done, and if she had symptoms of type-1, and the insulin was obviously working then the temptation would have been to assume that she was a type-1 and carry on with the insulin treatment.
