Aggressive Cat

Question
Hi,

I have a female cat named Smokey.
She is 9 years old, fixed, not declawed and indoor only.
She goes to the vet once a year and is completely healthy.

I got her when she was 6 weeks old, I also adopted her sister (Jen).
They were found in a box with their dead mother and 2 dead siblings when they were a few days old.
They were put with another mother cat and her kittens but she stop caring for them when they were 6 weeks.

I also adopted a dog (Nikki) almost 7 years ago when she was 8 months old. Smokey & Jen were about 3 years old.
At first Smokey would attack Nikki all time time but she was still friendly with Jen and me.
After a few weeks she got used to Nikki but still didn't want her close, she would hiss/scratch at Nikki if she got to close.
After about a year she started to let Nikki smell her and stopped hissing/scratching at her (unless Nikki got to hyper).

Smokey doesn't like other animals and is not the most social cat.
She used to be super close with her sister, they were never apart but when they were a few years old they started to ignore each other.

Jen is a very nervous cat and doesn't go near strangers, she only like to be with me. She loves to sit in my lap or lie next to me, she follows me everywhere.
She hisses/swats at new animals but gets use to them fast and after a week or two will cuddle next to a new cat or dog.

Smokey is also pretty nervous but she will go to almost anyone when she feels like being pet. She doesn't like to be picked up and will not lie on your lap. She will sit on the table/chair/couch and let you pet her until she has had enough then she will walk away.

I have been bringing them to the groomers once a year since they were about 6 months old and to the vet once a year since they were about 5 years old.
At the groomers they used to be very well behaved, I was always told that they were the best behaved cats.
About 5 years ago when I brought them to the groomers Smokey became very aggressive and was biting/scratching/hissing at everyone. They asked me to try to get her to calm down and I ended up getting bitten.
Since then she has also gotten aggressive at other groomers (I tried other groomers and recently have been bringing her to a private home to be groomed) and once at the vet.
She also started to be very aggressive to Jen after they came home from the groomers, I make sure they use non-perfumed shampoo.

6 years ago (about 7 months after I got Nikki) we moved from Montreal to Ottawa. Smokey becomes aggressive towards Jen & Nikki and hid most of the time for about a week.
After that she become normal again.
About 1 year after that she started attacking Jen all the time (when she walks by, when she gets to close). Sometimes they will be lying next to each other on the same couch and Smokey will jump Jen, Jen runs off and Smokey is sitting there with fur in her mouth/claws and start cleaning herself.
More recently Jen has started to fight back, if Smokey is coming near Jen will swat at her and run away.
Sometimes Smokey will come over and start licking Jen's ears and Jen will run away (I'm not really sure what is going on in this situation).
Also sometimes they seem to like each other, if one of them gets hurt or upset (usually because they got wet) and make a sort of sad meow or whine the other cat will come running.

Since before I got them I had been doing pet-sitting, mostly dogs & small animals.
In the last 4 years I started pet-sitting cats also.
When a new dogs is here (usually not more then 2 weeks) I will barely see Smokey the whole time.
Sometimes with the calmer or smaller dogs she will appear after a few days but not go near them without hissing/scratching.
The first time a new cat was here Smokey attacked him anytime he was around and spent most of the day up high.
After that I turned a room in the basement into a "Cat Room" and that is where cats I pet-sit for stay.
I figured it would be better since the cat wouldn't be near her/where she could even see it and it's not in an room that she ever goes into/not her territory.
It didn't works and whenever a new cat is in that room she will stalk outside the door, attack Jen for no reason (she will actually go after Jen instead of only when she comes near her), attack Nikki and she hisses/scratches/bites at me.
So far no one has gotten really hurt and I try to stay away from her but she seems to be getting worse every year.

Me stopping pet-sitting is not an option as it's the only job I can do and I need the money to live on/support my pets.
I am unable to leave the house much so pet-sitting in the animal's home is not an option either.
I only take one animal at a time and I keep dogs in one of two areas and cats never leave the room (except when dropped off or picked up and only in a carrier) so there is lots of space for her to go where she won't be around the new animal.

From what I have read I think Smokey's aggression is either fear or territorial.
I knows it's not health related because I have been to the vet and had her all checked out.
What can I do to help Smokey calm down or feel more secure?

A little more about the living arrangements:
There is also my parents, my younger brothers (19 & 6) and every few weeks my grandmother living in the house.
Smokey like to hang out in my parents room and gets attention from them and occasionally gets pet by my grandmother.
They haven't had a problem with her since they are not around her much and when they are it's not around other animals/during the problem situations.

Smokey & Jen share a litter box, they have never had a problem around the litter and I can't put a second one as there are allergies in the house so I only have one spot that litter can go without causing a problem.

They also share 2 food bowls (2 bowls in one stand), I have tried places bowls apart but one never got touched so I took it away.
They have never fought near the food but don't eat at the same time.If one is eating and the other one comes they will stare at each other and one will walk away until later.

They also share a water bowl with the dog.
This is there choosing not mine.
They have a water bowl near their food but they never use it.
It the same as the dog's bowl just a little smaller but they always drink fro the dog's bowl anyways.
It is basically the same as with the food bowls, they all take turns and don't go at the same time.

Feel free to ask any questions,

Thanks,
Jessica

Answer
Jessica,

It sounds to me like there's obviously a bit of tension going on between the cats... I'm thinking that a pheremone collar for each kitty would likely help ease the tension. Petsmart sells them for about $15 and they're good for roughly a month. This is well worth a try because the pheremones in these collars are the synthetic version of the cheek pheremones that their moms produce to help keep their kittens calm. These pheremones help calm cats for their entire lives.

I would also suggest that you consider modifying your pet-sitting so that you either have a separate area of your home for the pet-sitting clients or you care for the pets in their own homes because cats are territorial by nature and consistently bringing in new animals for periods of time on a consistent basis can be stressful for them. When cats are stressed they are more likely to have behavioural and health issues so minimizing stress is key!

Please keep me posted as to how these suggestions work towards changing the aggression issues with your kitties. We can certainly look at modifying our solutions if they don't work for your cats as they have been suggested. If you have any further questions or concerns, please feel free to contact me again at any time - I'm more than happy to help in any way that I can.

Kind regards,

Ali