1 1/2 yr old sister kittens-1 is hissing at the other

Question
We have 1 old kitty-15 yrs-and 2 sister kittens who are 1 1/2 years old. We went through the whole hissing at each other thing after they came back from the vet from being spayed. But today I took the old big kitty to the vet. When we came back, my daughter let one of the sisters out, (the other sister is more mild mannered & has no interest in going outside). I had to get her back in fairly quickly as we had to leave & when I caught her & brought her back in, she hissed at her sister whom she normally plays & sleeps with. When I came back home tonight, she was still at it, hissing when her sister got near her-it's so strange & am not sure what to do. I'm concerned about the littler of the two, although she does seem to be holding her own. Did the more confident sister just decide that dhe's the cat meow because she's been prowling around outside & has gotten all cocky?  I just can't stand to see my little sweet petite being bullied!

Answer
Hi
this is a behavior known as aggression trauma and is caused when something happens which makes another cat afraid. often aggression etc are signs of fear.

This is a common problem especially after a trip to the vets. basically the hissing cat is afraid by the smell of even the other scat own fear after the vets. often this will subside by itself but in some case they need a helping hand in rebuilding their confidence in each other again. this is done by using the introduction method described here.

/introducing-cats.html

best wishes Kate