Holy shit... what?!?
What an amazing family story... I read every word. Completely fascinated.
In my mind, in the "Nature vs Nurture" argument, "nurture" always loses. I know too many stories like your brother, where someone just doesn't take what they are given, or stories where two people who are related but have never met, share an amazing number of traits, like your sister and her daughter.
To me, you can nurture anybody. But if it isn't in their nature, they won't take it.