Newborn kittens should be kept away from other family cats until they have been vaccinated. A newborn kitten has a delicate immune system, which leaves it vulnerable to infection from other cats. The nursing mother also needs to be left alone to take care of her kittens in a peaceful and secure environment. Kittens can be vaccinated when they are nine weeks old, after which they can safely interact with other household cats.
Introduce the kitten's scent. An adult cat will feel more comfortable and familiar with a kitten, and is less likely to regard it as an intruder if you introduce it to the kitten's scent. Let your cat sniff a blanket the kitten was sleeping on or, when the kitten is a few weeks old, let them sniff each other through a gap in the door.
Ignore the kitten. The kitten can be placed in the same room as other family cats when it is around nine weeks old and has been vaccinated. Other family cats will feel reassured if you don't make a fuss of the kitten when it is in the same room as the older cat. Lavish the adult cat with attention to make him feel special and more well-disposed towards the new arrival.
Let the older cat approach the kitten. Don't try to force the cats together. Supervise the interaction, but don't be too alarmed if the cat is a little aggressive. It's normal for adult cats to hiss, spit or even swipe at a new arrival, but most adult cats will not seriously attack a kitten, according to the Safer Pets website. When the kitten is older, it's best to let the cats sort out territorial issues between them and not be too overprotective if the older cat gives the mischievous kitten an occasional cuff around the ear.