I have two cats, sisters from the same litter. I got one a year before the other. They have been in my home together now for over 6 months. The one that came later kind of pesters the other... to the point of messing with her while in the litter box. It was alright until they got neutered and now it is worse than ever. The one that pesters is so mean to the other she won't even come out of the room. I had to bring the food and water to her and lock the other out. Then later on lock the mean one up and let her roam the house and use the litter box. The mean one will even go find her where she is hiding and attack her for no reason. She is so scared of her sister she hisses and her hair rises if she even thinks she is coming... I've tried the water bottle and loud noises to let the mean one know she is doing wrong. They just got back 5 days ago from getting neutered... What do I do?
my advice would be to separate the cats for about a week if possible and keep the more aggressive cat confined to one room. make sure you still spend plenty of time with her so that she doesn't feel neglected. this will give both cats time to recover from their own operation and to readjust to a calmer quieter area without fear of attack etc. then the two cats should be slowly reintroduced using the training method here /introducing-cats.html.
It may take some time but is essential if you are to have any chance of a harmonious home.
the problem is that two female cats together often do not get on. i myself had to give one of my female cats away years ago when a new female cat came to the home and they were just not happy together, well i didn't give her away actually she found herself a new home down the road and lived their happily for many years.
Hopefully the training over a few weeks period may help the situation but i am afraid their are no garantees that they will ever get on. the best you may have to hope for is that they tolerate each other. Some cats are just like this I'm afraid.
Anyway i wish you the best of luck; and hope it works out well