Yep same here. I know a few families where siblings have had very different lives despite having the same childhood, same parents, same family experiences, etc., so basically started from the same point.
Same with my own brother. Don't want to go into too much detail, but our adult lives couldn't be further apart in terms of career, business, home, marriage, family etc., yet we both had the same upbringing, both inherited the same amount at the same time, etc. The only differences are a six year age gap and different secondary schools.
My OH's position with his sibling is very similar too. Again, same family background, but in his case, there's only a 2 year age gap, and they went to the same primary and secondary schools. Yet, their exam results are very different (one bothered to do homework and study, the other didn't), one was "into" the opposite sex earlier and made poor choices, the other didn't etc.
One of my clients, in fact, the most successful who sold his business for several million pounds, took risks and started his own business, built it up from nothing, and ended up employing a few hundred people. His brother worked in a supermarket, and even now, still works in a supermarket. Again, exactly the same parentage and family upbringing.
So, I do think people over-think the "benefits" of family and background - it really doesn't dictate your future life and that lots of other factors, choices and events have more influence.