Why must JB be the only one who'd hated boarding school ? His sister hated school-----full stop, and had been dismissed from one. Hundreds of kids hate school but it doesn't mean to say that they become murderers.
Children are sent to boarding schools for various reasons and NOT because their parents hate them or wish to see the back of them. Even if the brother and sister had been blood-related they would still have been sent away to school in order to give the children the best education that they could. It's what parents do when they're in such a financial position to do so and has nothing to do with anyone else.
Both were old enough to have understood about adoption when June told them both,together. It's just a pity that she hadn't explained in the same terms as the majority of new parents would have done,the explanation having been made in a matter-of-fact brusque manner,with no further questioning.
JB took it in his stride,Sheila hadn't done and probably wanted to ask questions but wasn't allowed,so whatever she'd wanted to know lay festering for the rest of her life and affected her greatly,hence it being so long in making contact with her blood mother not long before the death of herself.
All those years of suffering along with everything else she had to bear took its toll in the end,as it would with many who had to endure what Sheila did in her short life.