Dueling brothers

Lately the boys have been playing more and more together. Jonas tries hard to fit into Soren's games which now include fighting superheros in his undies in the backyard. ("Mom, why does catwoman have such big boobs?" Ha, that one cracked me up the other day.) But, along with the playing together has come the fighting together. Soren is a tattler. Constantly with the "Mom, Jonas hit me. Mom, Jonas got out of bed. Mom, Jonas grabbed a knife and tried to lob my ear off with it." It's whiny and annoying. And, most often it is not the whole story. The whole story would be that somehow Soren started the game that somehow ended up not in Soren's benefit and thus he tattles. Like, how did Jonas get that knife in the first place, hmmmm? And, Jonas, being small and younger and mostly defenseless retaliates in the only way he knows how. He bites and pinches. Really hard. Most of the time he'll give a warning saying "I bite chew," then he chomps down. Then Soren screams and tattles some more. I find myself playing referee already, most of the time unsure about what is really fair and worried that I may be unconsciously be favoring one over the other, who will resent it, and turn out to be bad person because he mom never sided with him. I'm pretty clueless about how to handle the fighting and I know it's only going to get worse. How am I going to deal with this?