Let me begin by saying I think you need to talk to an experienced specialist strata lawyer about this – but I think it could be worthwhile as these property managers may have overstepped themselves.
For instance, if they have voted to re-appoint themselves as property managers or have blocked attempts to take legal action against them, they might be in trouble.
It’s hard to know the answer unless you know the specifics (eg, if it is, in fact, a ‘board’, then this would be Company Title and comes under company law, not strata law).
And even if your brother has grounds for legal action, if I were in his position I’d be offering to sell the unit to the property managers at above market rates just so they can avoid the great trouble he is about to rain down on them.
Apart from that, strata litigation is one of those areas of life that even when you win, you often lose. And that’s why, ironically, your brother needs to talk to a lawyer asap.