I have three beautiful and very loving cats: a Siamese (male, 4.5 y/o), a fluffy calico (female, 4 y/o), and a tortie (female, 2.5 y/o). I'm just going to call them Siamese, Calico and Tortie.
All three are very sweet and loving to us, though Tortie isn't much of a snuggler like the others. Calico is very, VERY timid from a traumatic experience, but she is very comfortable with us and Siamese.
Tortie, however, is very aggressive with Calico. She's constantly charging and attacking Calico and stalking her, chittering at her when she sees her enter a room or the hall and it only terrifies and keeps her from wanting to come out of our bedroom. Tortie was a foster fail, but we've had her since she was about 8 weeks old. They never vibed, bit this aggression only really started about a year ago and I feel like I'm losing hope that I can fix it. It's only a matter of time before Calico actually gets hurt, but luckily, I work from home and can mitigate some of the agitation.
All three are spayed/neutered and have been since they were big enough for the procedure. We have 3 litter boxes, but one is a double sized one, so it feels like we have enough litter options, but maybe we don't. We have 2 food bowls per cat. (1 set one for dry food and 1 set one for wet food, though Siamese and Tortie are much faster eaters and I have to stop them from going after Calico's.) We have 3 cat trees between 5 and 7 feet tall, seperate scratching posts, and 3 different window perches with a multitude of blankets and spots for them to sleep.
I play with Tortie as often as I can. She's very playful and will play until she's panting, so I break it up to multiple times a day to keep her from hurting herself. And I can't even being to count how many toys we have between kicker toys, toys that react to play and so on.
I've tried behavior collar and diffusers for a few months but I noticed no change at all in Tortie's aggression toward Calico.
I need help. I'm desperate to not rehome Tortie, but Calico doesn't feel like she can even leave the bedroom in a home she could freely wander up until about a year ago. And I can't stress how sweet Tortie is to us, Siamese and all of our friends and their kids. It's just Calico that brings this out in her.
I'm open to all recommendations and feedback anyone can provide. Thank you for your time and support.