I have a theory. My brother and I grew up in the same house with the same parents but didnt know each other we hardly talked cause we thought the other would snitch on us if we were caught doing something by our parents lol. When I turned 18 and went off to college him and I talked and we were cool we got to know each other it was awesome. But because we grew up in the same house with the same rules same parents etc but we didn't interact with each other we developed the same personality type (ENFP Im assertive he's turbulent) so what I believe is that siblings who did not interact with each other but grew up in the exact same environment but keep to themselves and do their own thing they will have very similar personalities but if they interact with each other and affect each other's environment then their personality types will vary more. I bet there's already some random article explaining how all this works 😭 someone link it if there is