How to introduce kittens into a house with cats?

[Sneaky Cheetah]

I have two cats, and two 10 week old kittens.

The cats are brothers and around 13 years old, they get along most of the time but do occasionally fight.
The kittens are brothers, and are of course getting along amazingly with each other.

The problem is when we try to introduce the kittens to the cats whilst the kittens are very friendly and curious to meet the cats, they are met with hisses and growls from the cats.

We have done the technique of keeping the kittens in a separate room for the last 2 weeks, and allowing the kittens to eat next to the cats at feeding time. When they eat together they usually just ignore each other unless one kitten goes too close to the cats bowl.

At other times the cats have been shown the kittens in many different rooms in the house, and we let the kittens out to play when the cats are out so that they can get the scent around the house.

After two weeks of following various instructions from friends, vets and the internet we still have no luck in getting the cats to accept the kittens presence. So far there has been no violence, only hisses and growls, and looks of sheer terror from the cats (we don't know why they seem so afraid).

What are we doing wrong??


Additional details: the kittens aren't spayed/neutered yet. Both the cats are brothers from the same litter, and the kittens are both brothers from the same litter (although we think one of them might be a girl ((we havent taken them to the vets at all yet)) )