HELP REDUCE CONFLICT IN A MULTI-CAT HOUSEHOLD
For the first few days (at least) of a new kitten's time in your home, keep the cats separated from each other. Let the kitten have free roam of a closed room in the house. The adult cat will hear the meowing and sense the movement from behind the door.
2. Let Them Sniff Around
Place your kitten in her carrier in the middle of the living room and let the adult cat sniff around. Don't be afraid if they hiss and bat their paws or snort and hunch their backs. That's to be expected, and it's okay for them to do this. Try to give them treats or other items that help build positive associations.
3. Love Them Equally
Keep the cats separated in crates or different rooms until they accept each other, which can take a few weeks. When they begin to tolerate one another, make sure you play with them equally and show equal amounts of affection. You don't want either one to feel jealous of the other.
How to Handle Neighborhood Animals Causing Stress
Cats can especially get stressed by neighborhood cats, even if they never come inside the house, and they may take that stress out on other household cats. Seeing a feral cat wander outside can be enough to cause your cat stress. Try closing the blinds or even using motion-activated sprinklers to keep feral cats away from your home.
Remember, decreasing stress can take time. If your cat is acting uncharacteristically, it might take a few weeks or more to build new habits and help your pet feel secure. But with patience and love, your cats can learn to get along again.