I think half of it is on the parents and half of it's on the kids. My 12th grade physics teacher had two kids, both of which I met. His daughter was a great and brilliant girl. Went to Yale, despite the fact that she is neither a legacy student nor going to some ultra prestigious magnet school (parents couldn't afford it). I never met his son, but all I know is that he is in jail right now for dealing meth. I'm not making this up. They were both raised by the same people.
Typically when that happens it has nothing to do with the parents themselves but how they treated the kids individually. I have some similar stories as well. Boyfriend's sister and he have differing issues. Raised by same parents, he turned out fine, but his sister is a wee bit insane. She got away with everything as she was the favorite, and still continues to get away with doing things now that I couldn't dream of when I was her age(she just turned 21, her parents are paying for her college and living expenses out of town out of pocket so she still has to abide by their rules unless she wants to pay for both herself.. which she doesn't).
My sister and I as well. My sister(mentioned above with said 7 year old child) and I were raised by the same parents. Divorced parents, Stepfather and Mom were the resident parental units. I turned out mostly fine(little eccentric.. people think I'm nuts) and my sister's been in trouble with the law quite a few times, busted for drugs. I had emotional issues growing up and required more attention from Mom in the form of counseling sessions, doctor appointments, and other various things, and it made my sister rebellious and jealous. She started doing stupid stuff for attention, and it worked. She continued doing stupid stuff as she got older.
I now get accolades from my mother since I'm 31 years old and not constantly asking her for money or to babysit or whining about everything like my 30 year old sister does. ><