considering the fact that AA has been officially acknowledged as Orb's Vessal in their military forces ranks should be given to their crew according to military rankings. This just makes them more official especially in the chain of command they are probably the flagship and report to know one while they can still lead the Orb forces. You need a ranking officer to direct a military force and therefore since Cagalli won't go to battle she appoints her brother and those who she trusts the most to lead her forces against Dullindal.