I know this is very upsetting to you because hese are your babies.
Dont worry a bit about hissing - it is completely normal.
It is absolutely normal for your resident cat to vehemently object to any newcomers. All her hissing means is:" Hey, who are you? You smell different!! I don't recognize your scent and I am head cat around here - so watch it!!!"AND the newbie will know that and expect it.
This is the way cats act when they are first introduced.They have a specific language the use to communicate with each other.
You do not need a behaviorist for cats. They are really quite easy to understand.
They do not rationalize, nor are they capable of in depth thought or reasoning.
Everything they do is basic, logical (to them) and makes perfect sense.
In a cat heirarchy a strong kitty will always try to take over and assume the head cat position. In addition, that is always subject to changes if cat one senses weakness in cat two. One is always the Bully and the other is the victim.
Cats DO work things out in their own way and they could care less what humans (or behaviorists) think. They only respond to the energy of another animal be it feline, canine or human. Punishment is useless and counerproductive because they simply do not understand being hurt - they would just try to avoid the hurter in the future but it would not teach them anything.(except fear)
Cats do not follow a pack leader the way dogs do but they do sense their human moms energy and respond accordingly - so if you are nervous, they will be nervous. If you are stressed, they are stressed as well.
Since they have been together a while there is no getting to know you phase. They already know what to expect from each other.
However, females can be little witches. That is where they got the term catty and why it is almost never used to apply to a male.
Cats do not recognize others by sight, they depend more on scent, so if some tumil is going on in or oudoors, they can sometimes take it out on each other and begin hissing and spitting.Something spooked bully so she is taking it out on victim
I know this sounds simple - because it is - just be a calm, assertive, loving energy in their lives. When they are at home , they will look to you to judge whether the environment is safe. If you sit there with a calm attitude - it will calm them. If they sense that you are upset, they will think there is something to fear so just be as nonchalant as possible and CALM.
I would not confine either cat because they will want to get past any closed doors - cats are like that and they will be overwhelmed with curiosity as to what lurks on the other side.Plus they would be stressed by confinement (which cats HATE). . However,victim must always have a safe spot to retreat to if bully scares her.If she prefers to stay in a room - that's fine - she may have made it HER territory.
If bully starts at victim - just say SHUSH and her very lightly on the head. That lets her know (in a cat way) that the behavior is unacceptable and she will understand that feline method of correction and not be hurt, upset or insulted.
The problem is that victim is not defending herself against bully who is probably head cat. This is causing her to disrespect and bully her. THe way you can help is to become victims defense. A cat will only learn from someone stronger that her who is calm, assertive and puts him in his place. As soon as he shows any sign of going after her , grab her by her scruff and say NO! like HISS then gently but firmly tap his nose. You must do this every single time you see it.
Just imagine how stressed you become when they fight - well they can feel your anxiety and it makes them nervous.
Actually if you ever watched cats in the cat room of a shelter you would see similar behavior but they eventually do work things out in an amicable fashion .Because they have to!
Do not yell, spray water or act upset, Just immediately calmly and firmly correct her . This will teach her that her aggression will not be tolerated the way another cat would treat her - except the NO! would be a hiss and the tap would be with a paw. When victim sees bully being stopped she wil gain confidence and begin to defend herself. That will learn bullys respect and she will leave her alone.
Get a feliway plug in and rescue remedy at a pet supply to safely keep them calm.
Please let me know how things are going I will always be here for you.
Remember to stay calm at all times and I hope the cats will be fine.
Sincerest best wishes,