We've been having a really similar problem. We have a brother and sister (same litter) who we've had since they were 16 weeks old. They're about 2 now. A couple of months ago the girl cat (jinx) started taking swipes and hissing at her brother, Domino. Sometimes there's growling from her side too.

We can't figure out what's wrong at all. But yesterday I realised that when we open the back door for them to go out in the morning, Jinx is really hesitant to go out. Almost fearful in her body language, we hadn't noticed this until yesterday because we tend to go about our morning business before leaving for work but I wanted to see if another cat was coming into the garden. It turns out a really bold fox came into the garden and started to walk towards our open back door. I think he may even have been coming in and we didn't realise because we were upstairs getting ready for work.

I chased him off and sprayed him with water, hoping that this will make him more wary of our garden. But now Jinx is behaving as if she's fearful of me. I may have made some noise when I chased the fox out of the garden.

It's really upsetting, I've had a really close bond with Jinx all this time and she's very timid with me, aggressive and swiping at Domino (who doesn't react at all, just sidles off mostly).

On reading your post, I think we need to take her to the vet and see if we can get something to try and calm her down and perhaps also do a reintroduction with Domino. What do you think? She's eating well, drinking and she does still play with us too when Domino isn't around. She slept on my legs last night for the first time in a long time too so perhaps I'm on the way to mending some bridges with her. Any tips or thoughts would be very gratefully received. Oh and we have Feliway plugged in too.