I'm an only child with four kids (my wife is the eldest of five). Our kids, two of each, are as different as night and day. Having said that, once in a while they're "peas in a pod". I strongly recommend you never allow them to line up together against you. For me, I believe it's a mix of genetics and environment. If you really want to see the genetic component though, don't look at the parents, look at the grandparents. Sometimes it's uncanny.