One kitten hates the other after they were neutered. What's happening?

This is a question on the Reddit.com website which I would like to also answer on this site. This is quite a typical problem actually. Domestic cats identify almost everything that they encounter through their sense of smell. Or to put it more accurately, they confirm the identity of the object. This happens as you can see when they eat. They sniff food because at close range their eyesight is not that great and therefore, they've got to identify whether it is edible and nutritious through its smell.

One kitten hates the other after they were neutered. What's happening? Image: u/jkamio

When a cat goes to a veterinary clinic for any procedure including a minor operation like a male neutering, they come back smelling of the veterinary clinic. And the veterinarian has probably used a sterilising agent on the site of the wound which also will have a strong smell.

These smells transform the freshly neutered cat into a complete stranger to the sibling. Whereas once they recognised their sibling as a friendly cat who they knew, all of a sudden, they are encountering a stranger who has invaded their space.

In response they hiss at their sibling which is disturbing to their human caregiver. They hate to see this agonistic behaviour among friendly cats.

But the smell will fade and the kitten when then once again become a sibling who they like. And perhaps the smell of the operation can be removed with a damp cloth. In addition, a bit of bedding used by both cats could be rubbed over the cat who had the operation to speed up the return of their body odour to its true smell.

It is nothing to worry about although it is concerning initially. Years ago, I had a brother and sister siblings who got on well. The sister fell into a pot of white paint. It was water-soluble and I immediately washed it off. I also washed off her body odour. 

Her brother no longer recognised her and hissed at her. She groomed herself fastidiously for about an hour to put her scent back and at that point he recognised her and the status quo was renewed.

There is another point worth making. Siblings when they are young are normally friendly towards each other. When they become adults and independent, that friendship may disappear as they become competitors for resources.

That's the wild cat behaviour which looks peculiar in the home of their caregiver. There is no need to be independent-minded when they are both being looked after but of course it is instinctive. They may get along but they may not any more.