In Claire Powell's book she speaks of the time when both children were told of their adoption,which was straight and to the point then left at that.
I doubt if JB had felt/displayed any jealousy regarding Sheila's meeting with her birth mother. Neither siblings appeared to have shown jealousy about anything,simply because they'd been brought up fairly and equally.
Adoption is deeply emotional, just because Jeremy didn't react or show any emotion doesn't mean he didn't suffer. It's a huge truth to swallow, no doubt he bit his stiff upper lip very hard and eventually managed to bury it. Maybe it refused to be totally buried, poor little boy.
To tell a child 'straight and to the point and left at that' they were adopted may have been done with the best intentions but is horrific. They may as well have driven a bulldozer straight through those children's lives. Jeremy was then sent away to boarding school to deal alone with his 'secret' and his shame. Even if he's guilty my heart goes out to him, poor thing.