Kids are wearing social training wheels - they are still learning how to handle situations. This kind of a thing was a daily occurance in our cul de sac of 4 boys and 4 girls within 4 years of each other age-wise. Some kids are very vocal, some are physical, some are mean, others are timid. In our neighborhood there are two sisters who are sneaky and mean but you'd never see them doing it - but al lthe kids knew. They had many of us parents fooled - until we each saw something by accident that showed us how manipulative these girls were. They're all teens and pre-teens now and are going off in separate directions so it does go away by itself eventually.
The bottom line is that this stuff is going to happen - kids like adults are going to have conflicts. Actually it's nice to see that an older brother sticks up for his younger brother. Kids do "accidentally on purpose" hurt other kids (throwing the ball too hard) or it may truly have been an accident. Boys will push eachother down and slug eachother - but being boys they get past it and tomorrow they're playing and good buddies. That's how it's done in boy-world (it's a foreign land to us moms!!!!) In girl-world they'll be malicious and mean and gossip about eachother for years to come and in high school talk about an offense that happened in 3rd grade. Having a girl and a boy I'd much prefer the punching, slugging and knocking over - and then get over it by the evening.
They will learn better social skills as they get older. They'll earn not to punch eachother - and instead they'll progress to the phase where they'll tell the offender he's an idiot - that will pass too.
Don't get cuaght up in this - they're kids. 30 years ago they'd be playing ball in the sandlot and mom wouldn't be there to intervene - they'd work it out and go back to play ball again tomorrow.