Why does my cat turn on the others

Two years ago someone dumped a feral pregnant cat. She had 3 kittens all male. I caught them all and spayed, neutered and gave them vaccinations. The momma cat took off when her babies reached a certain age. I socialized the kittens, they live together in the barn and are fed daily. They sleep together and get along very wwll. Then two weeks a friendly kitten showed up, then another cat with a collar, a feral kitten, and every now and then a large male. Three days ago one of my socialized male cats started running off one of his litter mates. He will not let this poor cat come anywhere near the food, biting him and growling at him. He lets the other cats eat although every once in awhile he chases them. I am in the process of borrowing traps to do the trap, spay and neuter deal. Any ideas why my cat is running off his fellow litter mate? All of the vets I have called are no help. They tell me they don't believe in cat colonies, but in trapping and killing. I am overwhelmed. Any advice?

that's awful that the vets are no medieval! and anyway i dont' see what your problem has with cat colonies; it's clearly a behavioral/health issue. your cats are not a feral colony, they're basically housecats who live in a barn. i would bring both the aggressive cat and the one he attacks to a vet and get them checked out. sometimes there's no explanation, i knew two brother cats who were sweet to almost everybody but wanted to fight each other constantly. it could mean illness or it could just be personality, but hopefully the vet can narrow down the options (tho i'm leary of vets that still espouse 'trap and kill' mentality). good luck!

It could be that your one cat is visiting somewhere where he gets smells on him that the other cat is not familiar with.

It also sounds as if, now that additional cats are hanging around, he is worried about territory and food.

I also agree with the other poster in that your 3 males are socialized and are therefore not feral.

If all the cats were completely feral, they might be more likely to get along or at least not fight, but in this case the barn is their "house" and you are their "dad" and they are more territorial than they might be otherwise.

So far as dealing with the issue, I would be sure that you give your one alpha male sufficient attention and play so that he doesn't have boredom-aggression.

Also maybe separate the food a bit more. You might consider giving your alpha guy his own little area, or give the ones who are lower on the totem pole another area for their food.

It does sound like, with the addition of new cats coming around, your alpha's brother has ended up low man on the totem pole. I would agree to set up a seperate feeding area, though the alpha may just take that over too, its worth a try.
Territory and rank are things they just have to work out on their own, but if your low ranking guy is socialized and the food issue becomes a health concern for him, could he be coaxed into the house for a feeding time all his own?