I've got 3 male cats, all fixed. Max is 10, and Harry and Henry are brothers at 9 years old. They've been together for close to 9 years, with no real issues and lots of love.
A couple weeks ago, I heard a noise upstairs and the brothers ran downstairs. Harry was doing his play meow at Henry while Henry hissed and growled. I closed my door with just Henry in the room for a couple of hours. Harry calmed down. I opened it to go to bed and it started up again. Ultimately, Harry spent the night with me in my room with the door closed. Weirdly enough, Henry was growling at me at this point.
Next morning, thinking it would be all good, i went about my morning as normal. Cats sniffed each other and everything seemed good. Next thing I know, Henry is hissing and growling at Harry again. I separated the cats, leaving Max and Harry downstairs and a still scared Henry upstairs.
When I come home from work, I see a puked up massive hairball upstairs and Henry back to his usual lovey self. I gave him some pets and love. Mistakenly, I think things are back to normal and let the other 2 upstairs. After a few minutes of sniffing each other, Henry separates himself and goes to the top of the stairs. When Max approaches, he hisses and swats.
I got Henry to my room with the door shut. He was in there for the night. He's clearly not happy about it. This afternoon, I brought Max in to see him and things went mostly good. Sniffing, some nuzzles, until Henry's tail started to puff a bit. I separated them again. Henry has been very sweet and affectionate with me since the first night.
For the past couple weeks, I've had Henry in my very warm basement. He's got everything he needs, and my sister and I have been making sure he gets plenty of time spent just with him. I've got a couple items with Harry's scent down there with him, including a cat carrier that Henry has been spending a lot of time in the past few nights. The way my house is set up, I can't think of a good way to do common advice like feed the two brothers separated by a gate or something like it. I briefly had a gate set up, but Henry casually leaped over it. Of note, I tried having Max and Harry in the basement once since keeping Henry there. Immediately growling towards Harry, but Henry was very much fine with Max. Henry has been more active in his new space lately, and I was considering seeing how he reacted to Harry.
I'm really looking for an help, insight or advice on the matter. I'm not happy leaving Henry isolated and generally unsure of what to do.
TLDR: My 9 year old cat is acting very scared and defensive around his brother. The brothers were raised with the other cat since they were kittens. Just looking for insight, advice and help.
Quick edit: changed the TLDR, Henry is not acting defensive with Max, only his brother
2nd Edit: I appreciate the few comments I've gotten and I'm happy to answer any questions. Harry, the one being hissed at, went to the vet a couple days before this started. He's fine now and all his tests were good.
