Cats who were friends are now fighting

I have two kittens who are litter mates. One had a small lump on her head, and we took her to the vet to verify that it was a cyst and not harmful. We were only gone for about half an hour, and I was with her the whole time. When I came home, the cat who stayed behind is being really nasty to her and hissing and swatting at her. The cat who went to the vet doesn't seem to mind her sister, and is trying to hang out with her like they always do. 

I have them separated now, but I can't figure out why they are fighting. Will they eventually just stop? This is the only time they have been apart.

It's because of the smells your cat came back from the vet with, they are scary smells and the sibling thinks she is in danger around the other cat.

Either bathe the cat, use baby wipes on both, or do some scent swapping, or add the same smell to both, I've also taken a fabric softener sheet and gave each cat a couple swipes to change their scents.

Very common problem.

Yes this is very common!  My 2 are also litter mates and have done this after vet visits since they were kittens.  Noodles hisses even when they go together.  Mooch usually doesn't care so much.  For my 2 it usually takes about 3 days for them to settle down; but they never go beyond hissing.  I put a dab of vanilla extract on the back of each of their necks to help them smell alike again.  I also just keep an eye to be sure things don't look like they are going to escalate.  

I hope your kitties have calmed down now! 

Funny, this brings back a particular memory for me!

I've seen and experienced this issue many, many times, but these 3 were so dramatic about it, I thought I'd share.


This is left to right, Violet, Ziggy and Sebastian, all siblings and as you can see by Violet messing with Zig, all typical sibling behavior, lots of love, etc.

That is until their s/n appts.............Violet was by far the worst, the boys were very growly with each other, yet she was attacking them, it was so crazy I had to pull her out, then the boys did fine after a couple/few days, but Violet never got over it! Despite everything I did to fix the scents, etc. she would not stop going after them, and luckily was adopted first, then the boys went together, which I love always, so happy ending, but she was the only kitty out of countless kittens I've had to deal with this vet smells problem that wouldn't get over it!

Leave it to a female siamese right?! Feistyness

This wasn't to scare you into thinking yours won't, sure they will, maybe are already doing fine, just to let you know it is super common, nothing unusual.

It seems to be getting better, but the kitten who stayed home is still being nasty. She won't hunt her sister anymore, but still hisses if the one wants to snuggle or play. I really hope they go back to being friends, they are really cute together!

I have been keeping them separated, unless I am home to directly supervise them. It's not ideal for the humans or the kittens. is this necessary?

I think it will help speed up the process of adjusting again if together, but as long as you are sure one won't be mean to the other.

I think they should do okay together.

Put them together when you are there, and will be for a while so you can watch and listen that there is not too much drama with them, it will help you to feel better when you do leave them too.