bad tempered sibling

Question
Hi Ali! Certainly hope you can help. A few months ago we got two sisters from a cat rescue society in the UK. They were 9 months old and seemed to settle quite quickly: grooming, snoozing, eating and playing together. About a month ago one of them quite quickly became aggressive towards her sibling; spitting and lashing out. They now lead separate lives. Perhaps I should say at this point that this behaviour seemed to coincide with a vaccination visit to the vet. Whether this is relevant, I have no idea. When she comes through the cat flap she will immediately rub herself against everyone and seems to quite happily settle down next to one of us to sleep. The occasional stroke seems to be well tolerated but sometimes she will 'turn' for no apparent reason. Thinking this may be jealousy against her sister, I recently spent a whole evening cosseting her. She raised her head, purring, welcoming these advances and then suddenly her ears went back, mouth opened spitting very very violently and lashing out (plasters repaired the damage to my hand). The change in her was startling in it's rapidity. We are at a total loss to explain this behaviour. Her sister is a total delight. Any advice would be gratefully received as we don't really want to split them up but see no alternative at the moment as children (ours and others)are in and out of the house all the time and we worry for their safety. Thanking you for your time.

Answer
Jessica,

Has anything recently changed? It may not seem like a big deal to you, but things like different sights, smells, people, etc could trigger changes in her behavior. My vet has said that sometimes vaccines can have a negative effect on the personality of some pets. If the vaccines are the thing that you can pinpoint with some degree of certainty as coinciding with behavioral changes you may want to take your cat to a vet that has some experience with alternative therapies, more specifically homeopathy. My vet practices conventional medicine with homeopathy and herbal medicine, she gave me a remedy once for a cat that I fostered who reacted the same way to vaccinations, but I am at a loss to remember the name and strength at this point. It does seem to work well to change aggression and vaccine reactions that manifest as undesired behavior. In the meantime, watch and see if your cat follows a specific pattern before she lashes out and take note of it, as her pattern of escalation may well be important to develop appropriate behavior modification strategies. Do not do anything to reinforce aggressive behavior, if she looks like she is about to turn on you or anyone else you can put her in a room by herself to prevent the children and other family members she comes into contact with from getting hurt. This can serve as a time out or cooling off period for both you and her.
You can also try the use of a dollar store spray bottle that is filled with water and spritz her to negatively reinforce bad behavior. She will soon learn that you will not tolerate her outbursts.
Just to cover all of your bases you really should take her to the vet to make sure that the changes you see are not attributable to a physical problem. Failing the vet finding anything wrong with your cat you could see if an animal behaviorist might be able help you to implement a more comprehensive behavior program. I know that it is tough, but patience and consistancy as well as calm, assertive handling are the keys to bringing back the kitty that you know and love.
Please do keep me posted and let me know how your little one is doing. If you have any further questions, please do not hesitate to contact me. In the mean time I will try to chase down the name of that homeopathic remedy for you so that if your cat isn't sick I can give you something to try that may work.