Not saying yours are fighting for that reason, SalmonGod, sometimes kids just fight, but in general for excessive conflict its important for parents to look at which of their own behaviours might be exacerbating the problem.
I know my wife can be a bad example. She's not abusive or anything. She's just impatient and undiplomatic. Things get on her nerves really quickly, and she kind of just freaks out instead of looking for solutions. I see Hiro behaving very similar. Grim is going through the obnoxiously emotional 3 year old phase, and doesn't have much to do besides bug his brother. Hiro doesn't know how to handle it.
I'm really good at this stuff, on the other hand. I talk people, even small children, through conflicts and find compromises really easily. My patience and understanding is what holds the family together. That's not an egotistical statement. Everyone recognizes it. The problem is I'm just not around enough, due to work schedule, to help everyone cultivate good social attitudes and habits. All I can really do is clean up the mess.
I'm also very introverted, so I demand a good chunk of time at home to myself. I can't play the peacemaker role if I don't get that. So I often feel bad that I don't spend as much time with everyone as the ideal family guy should. My attention is wanted by someone every minute of every day, but I just can't do that.
But they're still really young, and they're exposed to plenty of good role models. I fought with my younger siblings about stupid things too when I was that young. I don't think I started getting over it until I was 10 or 11. My parents also had very little time to spend with all 4 of us. So long as I don't allow negative attitudes and habits to get deeply ingrained, everything should be ok. I expect things to calm down a lot in about a year, as Grim gets over the horribly emotional three year old phase, and is better able to include himself in the activities of all the older people around him.