The research on retention generally finds it to be helpful to achievement in the short-term, but not in the long-term, and detrimental emotionally, especially to boys, with significantly higher dropout rates in adolescence. It also tends to mask learning difficulties that would be better served with targeted remediation or accommodation, resulting in children with exceptionalities having to struggle unidentified for longer.
In this case, it is possible that, not only is the younger sibling taking the "non-academic" role to avoid poaching on older sib's territory, but she may be doing so as a coping mechanism, in the interests of a coherent storyline for herself that explains academic struggles that she intuits are discrepant.
So in addition to counter-messaging, obtaining additional data to explain cognitive and academic intra-individual differences may be helpful. For example, was her WJ cognitive uniform across subtests, or were some scores much lower?
