Having spent time with a family that is as the letter-writer describes, I think I have to disagree - Older sibling getting annoyed that younger sibling is reaching milestones before him, and pushing to "catch up" and the family treating it as 'only fair' that the older brother should have first bite at everything because he's older. It can start so young that that the family don't realise they're doing it. So the family I know thought it would be cute to show me tapes of them as baby/toddler. Older brother just getting the hang of walking, and in zooms younger brother into focus, walking quite confidently. Parents tell younger brother (a baby remember) to 'stop showing off' and focus on elder brother.

It carried on all through childhood and adolescence. younger brother worked really hard at school, older brother struggled but worked hard, but the younger brother didn't get support and praise he craved.

I came into this because I knew the elder brother first, and noted the dysfunctional dynamics to myself. I rapidly got myself out of there.