Siblings suddenly fighting

I have two cats, a male and a female from the same litter. They are now 6 years old and are usually inseparable. Two days ago they accidentally ran into my parents cat ( we keep them separated when visiting ). They got scared ( my parents cat just went outside. She wasn't aggressive, actually hardly interested ) and I think they blamed each other. This has happened twice before, last time they just hissed at each other for few hours, then it was over.
But this time they are fighting. They are both trying to get over what happened but everytime either one of them try to act normally the other one gets scared. And it starts all over again.
The past two nights I've kept them in separate rooms and rest of the time I have been very close to make sure they don't start fighting again.
At the moment the female is trying to move around normally but the male starts crying ( Threatening? ) when he sees her. And then the female starts growling and hissing.
Any quick solutions? We should go home very soon and I have a small apartment, I can't keep them separated there.
I should also mention that they have both been neutered.

Welcome to TheCatSite!

This is not an uncommon problem. I would try three simple solutions: 1) put a dab of vanilla under each cat's chin and at the base of each cat's tail. This will make them smell the same which will make them more comfortable together. 2) Buy and plug-in a Feliway Comfort Zone diffuser. This places a calming pheromone into the air that will de-stress the two cats. (You can buy this at most pet supply stores) 3) start giving them their favorite treats whenever they are in sight of each other. This way every time they see the other cat they have happy thoughts (ooh, yummy treat time) rather than scary cat thoughts.

Good luck!

I don't see any quick solutions if the usual vanilla scent thing doesn't work. You'll probably have to do a reintroduction, even if you have to find a way to keep them separated in the apartment. If that fails, a vet or behaviorist might prescribe medication to help them get along.

I've read about such cases before (redirected aggression breaking up friendships) that were solved with some patience and effort.

Tigger and Eightball (brothers age 8-1/2) started fighting with each other when my Bogart was in late stages of CRF and they knew he was terminally ill (Bogart was Beta cat and they were vieing for that position).

I separated them for a while and only brought them together when there were "happy times", such as play, canned food treat time, bed time. If they started in on their aggression, they both got time outs and removed from the happy time. I didn't give either one of them preferential treatment. It ended as suddenly as it began about a month after Bogart crossed.

Thank you for all your suggestions. I plugged the Feliway in one hour ago, I also got some vanilla, got a bag full of new toys and bought some new treats. Now I'm hoping for the best.
They aren't actually behaving quite as aggressively as they did two days ago. Right now my little girl (Vilma) is still scared of her big brother (Valtteri) who is acting almost normally.
Biggest problem at the moment may be that I can't be with them both at the same time. They are both very affeccionate and are never far away from me. That is until now.

Valtteri (male) has now started to go to his sister every now and then. They sniff each other and have even licked each others faces a few times. But only when he makes the move.
When they are in the same room, Vilma is not allowed to move. She stays on top of a wardrobe and every time she jumps down, Valtteri starts to "cry". Vilma is behaving quite submissively and she only jumps down after Valtteri has " given her permission". At least that's what it looks like to me.They stare at each other and Valtteri makes this come and play -noise. Then Vilma jumps down and Valtteri goes grazy.
Is it still the same fight or are what is happening? Can I leave them together unattended?

Satu

Sudden unexplained hostility could be a sign of illness in one of the cats. I would take them to the Vet for a check up.

Also were there any changes in or around your household that could account for this?

Originally Posted by satu

They stare at each other and Valtteri makes this come and play -noise. Then Vilma jumps down and Valtteri goes grazy.

Click to expand...

Do you mean that Valtteri attacks Vilma? If so, I would not yet leave them together unattended.

Satu -- I have a very similar situation in my house: 2 female, littermates, 8 years old. About a month and a half ago one of the girls was on the back porch, fixated on the neighbor cat in the yard next door. Her sister snuck up behind her, and a very serious fight began. I immediately separated them. Hissing and howling ensued all night until I separated them with a room in between them the next day. I took them to a behaviorist vet a couple of days later, and the one who stated the fight was diagnosed with redirected aggression. I was instructed to give them a “cooling off†period – to keep them separated but to change swap their living space every day so they donâ€[emoji]8482[/emoji]t get territorial. I use a towel to exchange their scents every day (they rub on the towel to release the pheromones).

I am now in the process of re-introducing them to each other, and itâ€[emoji]8482[/emoji]s going fairly well. I accidentally let them interact a couple of weeks ago, and they seemed fine around each other. However, a couple of days later they were fighting again. So, I had to start the “cooling off†period over.

It is hard when Iâ€[emoji]8482[/emoji]m with one cat and not the other, but I do try to play with both of them for about 30 minutes every day (but I spend more time with them than that, of course!). But, the serious play time seems to help.

Good luck – I feel your pain.

Vilma and Valtteri are again friends! I'm so happy.
I bought the feliway plug-in and vanilla. I gave them treats when they were together and played with them a lot. They were still very stressed so finally I gave them a cooling period of one week and after that started to slowly introduce them to each other. After a couple of days they could't wait to be together.
I was still worried yesterday bacause Valtteri suddenly ran after Vilma (and tried to bit her in her bottom) and she seemed stressed. So I didn't let them stay together after that.
But today Valtteri did the same thing and VIlma took it as playing. They wrestled and seemed relaxed.
So I want to thank you all for your advise. I was so lucky when I found this page (and so were my cats).