We have had our two cats (male and female - both neutered) for 3 years. They aren't litter mates but brought up together from kittens and are the best of friends.
Yesterday, we adopted a new kitten from CPL, a tiny inoffensive black longhair. We've set her up in her own room with everything she needs. The cats know she's in there and have exchanged glances around the door yesterday whilst we were settling her in. They don't seem keen!
Our male keeps sitting by the door making odd sounds, the sounds he usually makes when he is outside and a neighbours cat shows up! He doesn't fight but loves to sit under cars growling and meowing. He's still acting normally downstairs though, wanting cuddles etc.
Our female hissed at her and ran off and is acting very stand-offish even when downstairs. She's been hiding under the dining table and keeps running away when we try to stroke her. She's by no means a lap cat but she does love been brushed and stroked.
I know it's very, very early days but I'm so worried that we'll ruin the dynamic for the two we've already got. I've read up on introducing them but I think we just need some reassurance that we'll get there soon enough (or eventually)!