Sudden cat aggression - siblings

[JMoney99ca]

Good afternoon,

I am starting to lose it. I have 2 cats, brother and sister, 8 years old, lived together always. About a year ago, my boy cat got sick and it was my first experiencing trying to reintroduce them. The girl cat would hiss and growl whenever she got near him. Took me three weeks to get them back together. It was so traumatic for me i dont even remember how i did it.

So 7 weeks ago out of nowhere, the girl cat started hissing and growling and there was a chase ending with the girl cat hiding under a bed making horrible sounds. I stopped her brother from getting too close with a coffee can and squirt bottle and they have been separate every since. I tried the same steps as the first time. Kept them separate, allowed for cooling down, etc. nothing has worked. Here is what i have tried, mostly for 2 week intervals

Lily gets upstairs, walled off with baby gates and a sheet for no visual contact.
Stan gets downstairs. I make an effort to spend time hangin with them both and playing with them both
I have purchased:
Diffusers, new toys, calming treats, Jackson galaxy stress and peace maker formulas, spent an hour on phone with a paid behavorialist

I have kept them separate
Fed them on opposite sides of door
Given special treats to each while one is behind the gate successful with no hissing/growling for 2 weeks
Changed their locations to integrate smells
They eat each other’s food, use each other’s litter box etc

First time after 1st two week, i let Stan go into the room where Lily was and she was hiding under the bed. When he found her, she made those horrible terrifying growls and hisses and sounds. Spent another 2 weeks calming and keeping separate, then Stan jumped the gate while i was downstairs and i heard growling and went upstairs. I decided t ignore them and see how it went. He walked around the room and she kept her cool more or less. I even went downstairs to get my coffee....but then i heard running and when i got up there he was chasing her back under the bed so i squirted him and separated them again.

After each incident, they both go back to looking around every corner for each other after they see each other. Even after the treat exercise. I am so stressed and out of ideas. I live alone so the books that say play with them at the same time separately in the same room etc isn’t that easy to do. They both want to be around me so when one is with me, the other is generally upset waiting for me to come back. They are vocal and annoyed..lol. Lily destroyed the carpet by the door one day and Stan head bumps the door annoying my neighbors, both coupled with a sort-of howl

This issue is my house has hallways and stairs, making it hard to watch them all the time. If i open the gate, she is nervous to come down and when/if she does, once she sees him she hisses and goes back upstairs. once i open the gate, he wants to go upstairs so then she gets nervous and scared etc. its kinda like he wont let her go anywhere.

After a bunch of days of good treat sessions etc, what do i do next to try to integrate them. Do i let her his and growl at him? Usually after that, he walks away but always comes back a few minutes later or she will go looking for him.

Open to any/all suggestions!

Jennifer

[Cat-FAQ]

The following link may be useful for you: https://www.catchat.org/index.php/cat-b ... aggression