Thanks for considering my thoughts!
You said your brother was 24, right?
To be honest, I am also wondering why parents wouldn´t trust and expect their 24yo son to solve his issues without their help.
Ok. Maybe this is at least part of the problem: As an adult, he perceives unasked for attempts and advice as overstepping his personal boundaries, and possibly as disrespectful to his autonomy?
In my experience, people rarely are responsive to such criticism. They tend to get defensive (and, actually, I can understand why that is). I have come to find it more productive to ask them questions, to show them my interest, to search for the good intentions behind seemingly destructive behaviours.
A family is a complex system. Many (known and unknown, conscious and unconscious) themes and issues are intertwined and interdependent.
The behaviours that the members of the system display aren´t contextless - they are reactions towards behaviours of the other members and towards the paradigms of the system, and (oftentime surprisingly) even seemingly destructive behaviours turn out to actually be (more or less successful) attempts at stabilizing the system, while at the same time attempts at taking care of one´s individual well-being and existential needs. To reconcile these tasks simultaneously can be a great challenge (and sometimes the resulting strategies and behaviours appear to be downright absurd).
This is just one wild hypothesis (so take it with a grain of salt, it´s meant to be an illustrating example rather than my actual guess):
There is a reason why a man at the age of 24, lives still with his parents. There is a reason why he hasn´t moved out, and there is a reason why your parents haven´t yet asked him to move out. Both parties have an open or hidden benefit from this situation (or else they would have changed it). Thus, stability of the system demands them to maintain this state of affairs.
Of course, this situation keeps the young man in an inadequate child-like status, and his parents in an equally inadequate status of feeling responsible for him.
From the young man´s perspective, he can´t demand respect for his autonomy (because it´s destablilizing the system and threatening the hidden benefit of the parents), but neither can he remain like a needy obedient child (because that goes against his own need for growth, and also will earn him the reproach of being immature). Whatever he does, is "wrong". That´s exactly the situation in which a person will display irrational, absurd or paradox behaviour (e.g. verbalize his quest for autonomy in the behaviour and wordings of an immature child - he tries to reconcile the irreconcilable tasks which both are existentially important for him).
Of course, the parents are likely to display equally irrational behavours (because they are stuck in the same trap, vice versa).
Maybe this helps a little.