I have 4 cats in my household. A female brown tabby (oldest, 9 yrs), female black cat (7), female black & white short-hair & male black & white short-hair (both 5). All neutered and spayed shortly after it was available. We've had them all since kittens, from the local shelter, besides the black cat which was a kitten we found on the street.
It's a large house with 2 stories, litter boxes & sources of water for each cat on both floors. They all eat together on the first floor in the kitchen. We also have multiple cat trees on both floors.
We neuter/spay all the cats we've ever had shortly after getting them. We usually only ever get female cats, but he was listed wrong at the shelter, they posted he was a girl when he was a boy. We fell in love with him anyway and decided to keep him. So it's a house with 3 girls and 1 boy, one of which is his sister that he gets along very well with.
It's been a few years since then, and he is perfectly fine with 2 of the girls. His sister is extremely close to him, and the brown tabby is a bit unfriendly but never gets into fights with him and they usually hang out in the same room, even grooming each other on occasion.
On the other hand, our black cat who has a bit of a temper is always getting attacked by him. She's always been one to keep to herself and loves her personal space. She's never mean to humans unless you don't listen to her warnings, and she lets you pet her on her own time. She just HATES our boy cat.
She hisses and growls at him to stay away, and usually it works, but other times he starts chasing her down and jumping on her while she's screaming bloody murder. We usually stop it immediately by clapping loudly, yelling his name and locking him in the bathroom for a while, but he just goes back to doing it again next week. We're at our wits end.
They often tolerate each other, they eat together and stand next to each other while watching people do something, they'll watch TV with us in the same room, etc. It's just sometimes he pushes her boundaries, she starts growling, and he decides to attack. It doesn't happen more than once a week, maybe 2 weeks. He also has never, ever attacked anyone but her.
What should I do?