hi I had the same problem. I had a kitten name Benji and i got a knew kitten, benji would hiss and hiss, so what i did was put the new kitten in a room away from benjie, then I would get some thing that smelled like my knew kitten and give it to benjie to smell. He would smell and hiss at it so i put them together and they would hiss at each other but after a weeek they were best friends.