Sudden aggression against bonded cat


TCS Member
Thread starter
Dec 5, 2021
Hi There, hoping I can get some advice, my two cats who are brother and sister and bonded since birth have suddenly started not getting along. They are both fixed and have been living in our house for almost 7 years since we adopted them as kittens. The girl has suddenlt started hissing and howling at the boy and he is really scared and doesnt understand what is going on. I suspect he got spooked and jumped on her when shocked and hurt her and she feels threatedned by him.

We cannot keep them in the same room which is really hard. He really misses her and cries for her but when she sees him its as if she doesnt recognize him. They were so close and its so upsetting to see them both like this. We took her to the vet this week and he couldnt see anything wrong with her. She is still very affectionate with us an purrs like crazy when she sees us. When she sees him its like she doesnt know him and immediately starts hissing and howling. We seperate them as soon as this behavior starts. Does anyone have any ideas why this suddenly started happening and what we can do about it. Thanks!


TCS Member
Top Cat
Jun 13, 2018
Central FL (Born in OH)
Hi. Sometimes the cat who is being hissed at is the one who sick. So, it might behoove you to have the brother checked out by the vet next. I hope blood work was done on her and will be done on him as well. Just having a vet physically look a cat over isn't always enough to detect a health issue.

It could also be a new cat or other animal outside your home that the sister can see or sense and is reacting to that by taking it out on her brother, commonly referred to as re-directed aggression. It could be other changes either within or outside the home that is affecting her but not him. See article link below related to this matter, and another one that discusses more general aggression causes.

I don't know how long that has been going on, but if more than a day or so, you might start with keeping them completely separate for a couple of days and scent swap between the two of them so that they smell like each other. If that doesn't seem to calm things down, you might have to do at least a partial re-introduction between them. Also, a link below about the introduction process - some of it may not apply, but there are parts that will certainly help you come up with a plan.
Re-directed Aggression In Cats – TheCatSite Articles
Why Do Cats Attack? – TheCatSite Articles
How To Successfully Introduce Cats: The Ultimate Guide – TheCatSite Articles