With regard to the difference between the behaviour of family members in the outside world and at home. I think it is as simple as them having the power at home. With you being the child, they are automatically in control and looked up to. Until you grow up of course. At the core of all their behaviour is a deep insecurity IME, so to the outside world they try to look good. Inside the home, the mask comes off and they are themselves. They use one of the children/family members to make themselves feel better, and for that they need to take the other person
down a peg or two. The person who they feel threatens their self-image. In that way, they can feel confident that someone else is stupider, uglier, weirder, more useless than them, whaetever their own issue of insecurity is. A normal relationship with anybody will simply not work because of the massive hole in themselves. I don’t think it is intentional in most cases.
It is baffling, though, to me, that so many people are OK living a life without love or affection. To me, that is no life, I would rather not live then. But again, I guess they are also deeply insecure on the inside. If they'd step away from the dynamic (golden children, other family members) and their superiority, who and what would they be? It all just seems pointless to me without love, but maybe they just don't need the love, but are content with 'security'. That is my DHs theory anyway.
All this I'm mostly telling myself at the moment, because I am struggling with seeing golden children in my and my husbands family having the fullest lives superficially, and doing well financially, without people seemingly seeing through them. But obviously there are a lot of other people around who are also emotionally stunted, so they would not notice, and it is easier to acquire money if you don't worry about feelings or other people.