If the barrister thinks it likely that the court will
not consider the rules on 'joint criminal enterprise' to be appropriate, then the injuries inflicted by your brother can be separated from the actions of the guy with the knife. In which case, he's need to consider whether:
(a) the injuries inflicted by your brother were serious enough to constitute 'GBH' ; and
(b) if so, whether your brother's actions fell within the definition of 'with intent', which makes the GBH charge far more serious.
Paragraphs 45 to 49, here, show the definition of 'ABH' (which the barrister might seek to show is a more appropriate charge). Paragraphs 50 to 58 define the type of injuries for which a GBH charge is appropriate. Paragraphs 59 to 67 indicate the circumstances which give rise to 'with intent' being added to the charge:
http://www.cps.gov.uk/legal/l_to_o/offences_ag ainst_the_person/#P189_14382
Chris