这句话的意思是：我的兄弟就不会白白地死去了.
白白地死去是既成事实,整句话假设这种情况没有发生所以用have died in vain.
will加不及物动词的话是will die, 不可能是will died.My brother will die in vain是将会白白地死去,还没有发生.
have是完成时态,have died in vain过去完成.其实这句话更好的说法应该是My brother wouldn't have died in vain.虚拟语气,因为和事实相反.
in vain是副词性质的,白白地,无用地