You can have two siblings that are completely different. One who ends up a doctor, and the other that ends up a junkie.
I think a lot of it is just luck of the draw, though obviously good parenting should increase the odds for any kid to not grow up a scum bag...it's not fool proof.
A really good example of this - going back a few decades- is Billy and Jimmy Carter. One was an Admiral and President of the US, the other was a drunken loser.