Bullying within the family - when does sibling rivalry take a dark turn?

Here's an interesting article I ran across today as I sit at home battling the flu.

I think that this raises some interesting questions on how to identify and work with your children when the bullying is going on between them.

I have a 15 year old stepson who is fantastic at helping be a strong older-brother figure for his sisters who are now 8 years and 6 years old.   I've observed that even with their age gap that he sometimes still crosses that line into bully-dom.  Where I see more frequent symptoms of occasional bullying is between the girls.  Since that age differnce from the their brother is so great, sometimes it feels like we're raising a whole different set of children.  At any rate, most cases it is my older daughter (who is a fiesty red-head with a bunch of energy) who seems to push around her quieter sister.

http://blogs.psychcentral.com/family/2009/11/bullying-between-siblings/

What have your experiences with your brothers and sister growing up?  What have you observed with your own children?