Assuming your brother has earnings either as an employee or independent contractor then he MUST file his own income tax return.
The only way your mother can "claim him" is to declare him as a "dependent", though her legal ability to do that seems to be in doubt, given what you've written. To claim his as a dependent he has to live with your mother for more than half of the tax year
and she would have to provided him at least half his financial support both of which seem unlikely if he's really been living on his own for the last year.
Even if your mother can legally claim him as a dependent he still has to file his own income tax return and would receive his own refund. Your mother can't include his income and any taxes he's paid on her income tax return.
Tom Young
