Help with introducing new kittens!?

[Christian]

So here's the deal: I have been "cat-sitting" my brother's two 1-year old kitties for the last 4 months as he finishes his last semester of law school up. Me and these two cats have gotten extremely close. As a result, I began thinking about getting two kittens of my own in order to fill their void when they move back in with my brother. Yesterday I finally got my two, 12 week old long haired maine coon kittens; they are extremely lovable and are great with other cats. The problem is this: my brothers cats, who will still be living with me for the next month, are used to lots of attention and are very dominant and I am worried about my new babies getting along with them.

Before bringing my kittens home, I did my research, ensuring that my kittens have a room to their own until my brother's cats were ready to meet mine. When I brought my kittens home, I immediately placed them in their room where they stayed all day. Their food is on one side of the door and my brother's cats' food is on the other side of the door, so that both pairs can get used to one anothers' scents. Finally, this morning, the day after I brought home my cats, I decided to see how the two one-year olds would react to the kittens (they were secured in their carrier and were allowed to be inspected by my brother's cats) : one was suspiscous at first but later got over it and walked away; the other, however, is the noticeable more dominant one and let out one short hiss and would growl at times. I then let the older ones check out the room the kittens were in. Finally, I returned everything as it was before, with my brother's cats to roam the house as they please and my new kittens in their closed room.

My question is: what are some of your suggestions on how to better assimilate my new kittens with my brother's and, more specifically, how to get my brother's more dominant cat to accept the babies.

The hiss and the growling have scared me and have left me worried about the safety for my new kittens if they were to ever get near him.

PLEASE HELP!