There is no way I can speculate on whether or not your sister is being/has been abused. I am just presenting the possibility. Children who have been abused may suddenly display a shift in mood, for no apparent reason. Some children will begin throwing temper tantrums, while others will "shut down" and avoid talking to people altogether (or just the person who is abusing them). Some will be overly sensitive and cry easily, while others will engage in self-destructive behavior (cutting/drinking). All I wanted to say with the doctor portion of my post is that everything should be ruled out, including physical/sexual abuse. If the doctor notices bruises during his/her examination of your sister, that could be a warning sign. Of course, this doesn't take into account emotional abuse/bullying, which is why talking to your sister's teachers could be very helpful. Since you don't seem to know where this behavior is coming from, it's important to explore it from different angles and figure out how you can intervene (and agian, by "you," I mean your family, not just yourself)
Dismissing this behavior as an effect of "hormones" would be premature. Not every 11-year-old girl acts out like this - in fact, many do not. I wouldn't say it's "normal" for ANYONE to stab someone with a pencil, regardless of their age. If you conclude this is something you have to "wait out," then you could be ignoring an underlying problem, with abuse and bullying being only two of those potential problems. So if you're asking whether or not I agree with that poster, the answer is "no." I don't believe it would be responsible on your part or your family's part to shrug this behavior off and see it as a "normal" part of child development that can't (or shouldn't) be addressed.