Introducing a new kitten in an household can always be difficult, i remember the 1st time i tried introducing a Persian kitten to my Siamese mother cat. She hissed at such a high frequency, it scared everyone in the house and i had to leave the Persian kitten at my friend's place. We always wonder why we cant just tell the mother cat to take care of this poor orphan kitten.
After this bitter experience , i set out to find out what i had to do to ensure the same experience never happened again and it turned out to be very simple yet very effective. First and foremost one must realize no cat can be made to adjust to another cat or kitten in a single day. The process has to be slow so as make sure the distress levels are minimum.
Hissing is a common welcome given by all cats to new comers and its bound to happen almost always. But at the same time it will subside in around a couple of weeks. The mother cat must be made to familiarize the smell of the kitten. We can do that by making the kittens of the cat as well as the new kitten play together or by exchanging blankets or both. The key to the process is to make sure the mother cat feels calm and happy by providing her treats whenever the new kitten comes or so on. This is improtant inorder to prevent jealousy. Once the kitten starts playing with other kittens in front of the mother cat it will be safe to assume the cat is at ease and in a week or two am sure they will all be sharing the same domain. The important thing to remember at all times is to give it proper space in the beginning and then merge the spaces slowly together.
This worked for me and am sure it will for you as well. For any advise comment below.