Cat hissing after 4 months

Question
Thank you for your time. I've recently moved in with my (soon to be wife) and brought with me my cat of 4 years. She currently has 2 cats, one 3 years old the other ~1.5. All of them are spayed/neutered, though my cat is the only one declawed (not originally my choice.) The younger cat (Hunter, small Male) and my own (Minerva, average to large female) get along just fine. They occasionally wrestle and play, but in the end Minerva will grab Hunter and lick and clean him, and they get along fairly splendidly. My problem lies between my cat and the older "original" cat (Cassie, large female.)

Basically, I've lived here for 4 months now, and Cassie still has not accepted Minerva into the home. She hisses every time Mini (Minerva shortened) enters the room and will exit quickly. We have a fairly large apartment and there is plenty of space. After the first month, my assumption was that Cassie just needed time to adjust. She will still occasionally hiss at Hunter, though that's mostly when he attacks or chases her. It's now been 4 months, and though she still occasionally has an issue with Hunter, she still REALLY dislikes Mini.

I pet Cassie and she accepts me (and likes me.) I'm just wondering what to do about the two cats together, and if there's any way to resolve this issue. Cassie is temperamental (especially about food) even though we have separate bowls and litter boxes for each cat. I read that cats respond to a human hissing, and I tried that for a couple weeks but that just seemed to scare her further so I have since stopped. I have never hit any cat, and even though I've thought about it I have never thrown anything or tried to separate them physically. They do not get into altercations, but I'm still at a loss. If you need any further explanations I would be glad to provide them. Is there any way to get Cassie to stop hissing/growling at Minerva? Please help! Again, thank you for your time.

Answer
John,

Some cats just do not get along well together. On the other hand, a tense truce is a perfectly acceptable relationship between cats.  As long as no one is really fighting and just hissing, I would ignore it.  In fact, I think cats dream up ways of manipulating us humans by how they appear to get along or not.  From the sounds of it, they are actually doing fine (given their respective ages). If Minerva was used to being the only cat and Cassie is the alpha female (between her and Hunter), it is no surprise they are not getting along any better.

We always must be on guard to not lay human emotions or motives on cats as they do not think like we do.  Cats find their own ways to have inter-relationships, and we can do nothing about it, nor, in my opinion, should we try.

I have not heard about cats responding in any useful way to human hissing.  In fact, I will not even use the "shhhh" sound with cats in trying to calm them down as they may get the wrong idea and perceive that I am hissing, which is not a cool thing for me to be doing from a cat's perspective.

All in all, after a month, given their ages, I think things are going remarkably well, and the less heed is paid to Cassie's and Minerva's discussions, the better.

Please let me know if you have any questions.

Seasonal regards... Norm.