She is a bit young, but it happens, and its happening more and more. Infact, the average age of the menarche (the onset of menstruation) is 3 months earlier every 10 years, which is obviously of concern.
Re your sisters early onset of her periods. Please dont be alarmed regarding what I'm going to tell you, as this is something that just needs checking out. OK?
There is relationship between starting periods early, and the halting, or slowing of growth, which can then result in shorter stature than might have been expected of the individual.
If your sister were to be taken to the doctors, then your doctor at the very least ought to organise an appointment with an endocrinologist (a consultant doctor who deals with hormonal probs), or a paediatrician. They would then be in a better position to decide on a course of action, if any, to take. Sometimes, they prescribe growth hormone for a few years. Sometimes they do nothing.
Having started my periods when I was 8 and a half, and being the tallest girl in my class at that time, and now being 33 and 5 foot 2... it kinda makes me wish that this information had been know back then. Oh.. and I know about this not because of a lifelong grudge re being 'vertically challenged', but because I'm a health visitor with a special interest in teenage health.