This is usually caused by irregular periods or a hormonal imbalance.
Stable Periods of little change interrupted by brief periods of more change is known as punctuated equilibrium.
Can you be more specific? No offense, but if you know this little you will not be able to learn much here. Go to your library or buy a how-to book at the home center. It will explain it in detail and be a valuable resource.
The question is awkward and frankly a little sexist, even if that is not the intention. Why would anyone need to explain or justify that women are competent human beings?
Evolution.
punctuated equilibrium
Evolution of man.
Yes, you could be pregnant. To be sure see your doctor for proper diagnosis and treatment.
If she has started having periods and have sex then yes.
a model of evolution in which short periods of drastic change in species are separated by long periods of little or no change
Desert
Yes it is possible depending on her age. I know this family who little sister got her periods before the older one. The little one was 13 and the older one was 15. So it may or not happen. If your sister is between 9 and 16 then she can get them
no she doesn't kill anyone