Cat Suddenly Agressive Towards Sister Cat

[Trysh]

My friend has two cats from the same litter they are about 2 years old both spayed and suddenly one has become extremely aggressive toward the other. Any ideas why this could suddenly happen.

 

[rubysmama]

Do you know if anything has changed in your friend's household? New furniture? Different people? Anything like that? It's also possible that she saw or heard another cat outside and thought it was her sister.

Is the aggressive cat eating ok? Using the litter box ok?

Here's a few TCS articles that might be helpful:
Re-directed Aggression In Cats
How To Deal With Non-recognition Aggression In Cats
Is Your Cat Stressed Out?
When Physical Problems Turn Into Behavior Problems

 

[Trysh]

There is a big Tom cat that has been hanging around and everything is fine litter and food wise

 

[rubysmama]

Well then, I'm betting the Tom cat has something to do with it. Those articles may be helpful.

Here's another one: Six Surefire Strategies To Reduce Stress In Cats

And if things don't improve, your friend might have to try separating the 2 sister cats and doing a re-introduction. Hopefully it won't come to that, though. Here's an article on how to How To Fix An Unsuccessful Cat Introduction which doesn't actually apply, but might have some tips.