Dear Dr. Barchas,
Santa Fe, NM
My most basic suggestion is to go slowly.
Cats are territorial creatures. That means Sassy views your house as her territory. When you bring a new cat home, she will feel much the same as you might feel if a new person moved into your house uninvited. It could be a very nice person, but you probably wouldn’t want him there. That said, you pay rent or mortgage, which entitles you to your own space. Since I doubt that Sassy pays any bills, she is not entitled to veto your decision to get another cat. And, with time, Sassy should get over it.
I recommend that you set aside a room in your house which will serve as the new kitten’s home territory. The room should be comfortable, with food, water, a bed, and a litter box. Confine the new kitten to that room, but don’t let Sassy in, during the adjustment period. Be sure to give the new kitten plenty of love. Sassy and the kitten do not need to meet face-to-face at this point. Trust me, they both will know the other one is there.
Meanwhile, before they meet, take the new kitten to your vet. He or she will be able to offer more advice for smoothing the transition. More important, your vet will be able to assess the kitten to make sure that she is healthy. Kittens from shelters may suffer from a number of health problems. Some, such as respiratory infections, ear mites, fleas, and ringworm are major inconveniences. Others, such as feline leukemia, are very serious. These problems could spread to Sassy if you skip this step.
Once the new kitten has a clean bill of health and both cats are used to the idea of another cat in the house, you can open the door to the kitten’s room and let them meet. Do this very carefully. Cats always should be supervised during the introductory period. Although some sparks may fly, most cats ultimately learn to get along with each other.