Outdoor cat's beating up kittens?

OK, so this is what is going on. My grandmother lives on a farm and people are constantly dropping off kittens, that they found, or were going to be sent to the pound. Well in early may, she had two cats, each with a litter of kittens. A girl came and dropped off two kittens the same age as the kittens that were already at the house. The new kittens ( Patches, and Daisy) would nurse off of the two mothers and all was well, they treated them like their own.
Well just with in the last month or so, Patches and Daisy are no longer allowed anywhere near the other cats. Instead of sleeping on the deck in back with everyone else, they have to each up front on the porch, sleep in the vines up front, and basically aren't allowed to move away from the porch. If they come around back, try to go into the barn, or the fields, all the cats go after them. Even the cats that they used to nurse from, are backing them into corners, either just one cat or three our four after them, and beating the crap out of them, like claws and biting, to where they are bleeding.
Does anyone have any idea why they would do this? Some one suggested that they might be sick. Any other ideas? What can I do?

Yes, they could be sick and the rest of the cats are rejecting them.

Other ideas? The 2 kittens are not official members of the "pride". It sounds like your grandma has at least 2 female cats and their litters. Are they by chance related and are there other cats that she cares for? And did the 2 mothers nurse any kitten, regardless of they were their own, the other cats, or the 2 dumped kittens?

Cats in the wild form social bonds which are usually between family members. Related females hang out in prides while males, once they are mature, are driven out and often form their own groups called coalitions. Females in the pride will mutually nurse any kitten born to a pride (if they are able to). If a non-related stranger enters their territory, they are more often than not rejected, particularly adults or older kittens. It's a territorial thing, and its their way of promoting their genes. I'm frankly surprised that the mothers even nursed these kittens in the first place. Young kittens are often killed.

I lived on a farm for a long time and when I had kittens dumped on my property, I immediately removed them from the rest and found them homes. Even though my "farm cats" were fixed, they still did their best to protect their territory.

I have to give the lecture: If your grandma doesn't get her farm cats fixed, the population is going to get out of hand. I know the excuse about costing too much to fix all of them, but in the long run, she will spend more money feeding them and caring for them than she would getting them fixed. She should contact her vet and asked for a reduced rate to fix her cats. My vet would charge me his cost for the cats that wandered to my property.

She has at least 4 female cats. The two kittens that were dropped off, are from the same litter. When they were dropped off they were about a week older than one of the litters already there. They nursed on the other cats, slept in the box with them, played with them. Just with in the last month the older kittens, and cats have gone after them.
Most of the kittens stay on the deck at the back of the house, but these two have to stay up on the front porch, and if they go to the side of the house they are on them. The group of old cats back them in to a corner and just go after them. I don't get it. They treated them like they were their own up until this point.