I am sorry you are worried.
Actually what you are seeing is quite normal between cats.
Humping can either be sexual or dominance.In your girls case, it sounds like cat A is telling cat B "I am the boss of you! So stand down"! The reply "hiss" is cat B saying "OK, but don't push me"!
Try to ignore this behavior and let the cats work things out themselves.......I promise they will. Stay as calm and nonchalant as you can muster.
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 hierarchy 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. 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 counterproductive 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 parents energy and respond accordingly - so if you are nervous, they will be nervous. If you are stressed, they are stressed as well.
Once they are spayed, they will no longer be dealing with hormones but the stronger kitty will always be the pack leader.There is always ONE in a multi-cat household (I have five)!
You sound like a fabulous kitty "parent".
Your cats are VERY lucky to have YOU!
If you want to discuss anything further, just press reply and I will always be here for you.
Warmest best wishes,