This is interesting as the disorder is commonly known as a permanent and ongoing set of symptoms, behavior patterns, and mood changes that can not be cured. With that said, it must be taken into account that many doctors and psychiatrists are more than eager to slap a label on anything that they can not explain. What could be teenage angst or just your everyday maladaptive behaviors could easily slip by a clinician and present itself as mania or depression when indeed it is just hormonal problems, external strife, attention seeking behavior, or any number of inappropriate reactions to life's problems. I once had some green stick doc try to tell me that I may have OCD because of my barrage of somatic complaints. He was totally out of his league in labeling me with something so utterly ludicrous, and I quickly found a more educated professional to help me manage my disorder. I soon discovered that my somatic complaints were just a result of the depressive phase I was going through. Anyway, my point being, if it were truly bipolar disorder, it wouldn't just resolve on it's own.
My other point is quite clear, just because a clinician labels you with something doesn't necessarily mean that they are right. Read up on it and you'll see that once you have it, it's your friend for life. However, I'm glad that your step daughter is doing well without having to be on the bipolar med-go-round. For me, finding the right medication only to have it stop working after a period of time is the most frustrating aspect of having this disorder.
I do believe that there are a select few that can manage the ups and downs of the disorder, but it is very uncommon and is an extremely difficult task if you are truly bipolar. I've dropped my meds on multiple occasions only to come crawling back to them beaten, battered, and hopeless. In regards to your step daughter's condition, I don't know if I'm motivated to share all of this information due to skepticism or just sheer jealousy that somebody can weather the storms without a consistent, effective, and buoyant medication life raft.