My brother acquired his British citizenship/Passport abroad, he never stepped a foot in the UK before, he is coming to settle in the UK next week, can he work instantly the same day he arrives or he will have to wait till he gets an NI number?
Sort of. He can work, but he will be emergency taxed. This is a higher rate than normal tax.
Apply for an NIN as soon as he lands: https://www.gov.uk/apply-national-insurance-number