My cats won't stop fighting

In January, I got two kittens. They're 11 months old now and settling in pretty well, both the male and female. The problem is, I already owned an 8 year old female cat (Tammy). I introduced her slowly to Dolly and Benny (kittens) and everything went super well. About a month after the settling in process and introduction was complete, I noticed a bizarre and disturbing change. Tammy gets along quite well with Dolly, and loves to wrestle, chase, and play with her with no odd sounds. But she DESPISES Benny, the male. Benny always tries to play with her too, and she snarls, growls hisses, and cuffs him round the head and takes out chunks of fur. Benny NEVER reacts back to this, but instead keeps pushing his luck and trying to play with her and follows her around even though she clearly wants nothing to do with him. Naturally, I decided to separate them. I keep Benny and Dolly in my room, and tried to keep Tammy out. The problem is, Tammy clearly wants to be in the room. She whines and cries and scratches to come in, which totally dumbfounds me as I thought she'd want nothing to do with Benny or any room he's in. I let her in the room and all is fine until Benny moseys over to her and sniffs her face, prompting a snarl and a swat. Separate, rinse and repeat. She's totally indifferent to him until he wants to play and interact. All the cats are fixed. None of them share litter boxes, food bowls, toys, or scratching areas. Each have their own separate areas of the house, but they WANT to mix with each other so it's difficult to keep them separate. I'm totally at a loss. I don't want to rehome Benny. He's my sweet boy and is absolutely TERRIFIED of every single human in the world except me. He's super attached to me and I could never bear to give him up. I'm mystified as to why he doesn't react back, when Tammy is clearly furious with him. Any tips or explanations for why Tammy would hate the male and love the female, even with everyone being fixed?

In my experience, males and females tend to get along better than females and females. It's hard to know why she is reacting this way to him without knowing more about his behavior. Sometimes when there is an age difference between cats the older cat will not like how the younger ones play and may become annoyed especially if they are very young. My cat Treasure is not even 2 years old yet but she gets very annoyed when my younger female tries to rough play with her. Sometimes kittens will jump on top of another cat and other kitten behaviors that older cats aren't as tolerant of. Since both kittens are 11 months old it's hard to know why your older cat would be ok with one and not the other unless they interact with her differently. Does the male kitten play rougher or is he more hyper?

Even if they do get along sometimes and she wants to be near them that doesn't necessarily mean that they don't still have issues with each other. It could be a territorial thing, sometimes a cat that is used to being the only cat in the house will have difficulty accepting new kittens, but if she gets along with the female kitten I would think that it is more likely that there is something he is doing that she doesn't like. His behavior may be very innocent and may not be something that you would think would cause her to react this way. My cat Treasure will cuddle with my younger cat Shadow but when they play together they play very differently and don't always get along, partly because Shadow is very needy and clingy and doesn't give her any space. They love to groom each other and sleep cuddled up together but sometimes it does happen ( very often) that all Shadow has to do is go near Treasure and she will growl and hiss. Shadow doesn't respect her space and won't ever leave her alone, unless I put them in separate rooms to "cool off" for a little bit. I truly believe that it is Shadow's clingy behavior that Treasure does not like.

I don't think that rehoming is something that you need to worry about. I would just give it lots of time. Sometimes it takes more than a year or two for cats to get comfortable with each other. Having them separate and slowly reintroducing them when you are there to watch them is a good way to start. I would try to pay attention to their behavior, sometimes it can take a while to figure out what is causing a cat to react negatively to another, sometimes it is easy to overlook the most obvious thing.

I have 5 cats who all get along.....except one of my females & a younger male who joined the family about 4 years ago. Same dynamic of the younger male repeatedly coming up to "play" with my girl who wants nothing to do with him. Now they have developed something of an understanding where they just avoid each other. It stressed me out for a while, but they have worked it out....not in the way I would have hoped. Ideally they'd all just be one big happy family, but I suppose this is the next best thing.

His behavior, I think, is what puzzles me the most honestly. He's so gentle, never raises a "paw" in his own defense, and only wanders up to her so he can sniff her and that's what sets her off. If he comes too close, she'll growl a loud warning which he will ignore, and attempt to cozy up to her, prompting a snarl/hiss/swat. Benny HAS tried to romp with Tammy - the most he's attempted is to jump on her and try and roll around like he and his sister do. Needless to say, it was not a pretty scene. She screamed and sprinted out of the room. But his behavior is no different with Tammy than with Dolly, the other kitten, who has chased and jumped on Tammy and gotten a happy response back. Tammy will actively engage with Dolly with no threatening sounds or gestures. However, I do work during the day and it's possible he's bothering her in a way I'm not aware of. I do keep them separate now during the day, but it's possible that back when everyone got along and I let everyone mingle on their own that he started SERIOUSLY irritating her differently than Dolly. 

I think Benny's #1 problem is that he refuses to give her space. When Benny decides he wants to cuddle with Tammy, he sets his mind to it and won't let up. Dolly doesn't try to interact with Tammy as much, maybe that is why Tammy is more tolerant. It breaks my heart! Benny is so docile and Tammy just hates his guts. I'm so relieved to hear that you think time may actually help. 

Have you tried a Feliway plug-in diffuser? They emit calming pheromones that really should help. If plugged into the room or rooms where the cats spend most of their time, you may see far less aggression very quickly.