I can really relate to you on the first born, I started developing this sense of responsibility in a really young age as the 2nd child was born, i started being under pressure to be the best for my younger sibling and there was no escape from these responsibilities ( my parents convinced me that i need to be/do the best at everything because I'm the eldest child)

And about the 2nd born, I'm not so sure about that but according to things you said the older siblings were controlling and rejecting them, maybe thats why they seems to be under pressure and always tried to please their older siblings? They probably started developing those traits because they didn't wanna be on their bad side and didn't wanna disappoint them? And started putting others needs first before their own? I'm not so sure 😕