I deal with this issue with my clients quite often. Same-sex fights are very common. There can be many triggers that set off dog fights, and sometimes the only way to tell what the triggers are is through direct observation. There are many possibilities. Since you mention this happens when your brother arrives, it could be they are very likely competing over who gets to greet him first. Also, if they are both very excited, this state of arousal may put them up for ''discussions''. Is one dog much older the other? At times, a younger dog senses the older one is getting older and frail and may attempt to assume a more assertive role.
Smart and Fun is very right: always keep a watchful eye on them. I know too many owners who have decided to let their dogs handle it and an expensive vet visit later have regretted it. Best wishes!