And they obviously keep it quiet till they were about 7 years of age. This is written in CAL's book. So the teasing had occurred before they were 7. That attitude toward the two Bamber children can't have improved as they grew older. A cuckoo in the nest doesn't go away as it gets older. No the cuckoo displaces the natural offspring from their inheritance.
You're basically repeating what I said. Jeremy was told of his adoption just before he went to boarding school. We assume, unless he lied, he had no idea prior to that time, ergo, he'd have had no idea what his cousins meant by calling him a cuckoo before that time. I imagine, like most children, he'd have asked his parents................apart and aside from the fact that his cousins were some 10 and 13 years his senior, too far removed in age to have had any interest, BUT, the one person who MAY have named him thus was RWB, who had been, I believe, resentful of Jeremy from the off.
It's just occurred to me that Sheila was told some 3 years ahead of Jeremy. I find myself wondering if she managed to keep the secret from Jeremy, or, might she, when he was being particularly nasty, have let slip, that he was adopted.