There has to be a set of rules or protocols guiding the best man speeches. In fact, there are certain rules governing the best man speeches but giving a speech that is somewhat unique (like a best man speech for younger brother) you need to think beyond those generalized rules as they are basically the same for all best man speeches. So, if you are the younger brother of the groom, it's more about fun (and emotions) than rules when it comes to giving a best man speech. However, it is also important that you maintain the best man etiquette throughout your speech and other activities related to the best man.