LW doesn't really describe anything, though. All they say is that both siblings are going through normal adult milestones at around the same time - which is typical if they're about the same age.

Most adults in the US do buy a car. It's very common, once you marry, to adopt a pet and have a child. Is it possible the family is dysfunctional as hell? Sure - but you can't prove it from this letter.

(At any rate, my advice would be the same. LW should get therapy, stop talking about Big Bro, and reduce contact with family until/unless they can stand to talk to those people again.)