>>My cats who grew up together and have lived together for 7 years still do this every time they visit the vets, even if it's just an hour and they both go! They recognize each other my pheromones, smell. Once at the vets this scent is altered for awhile, even days. Since you got your kitty fixed his phermones or scent will permanently change and his sister will have to get readjsuted to her bother's new scent. It will take a week or two, but they should settle in after that. For just a short vists without surgery, my cats do this to each other for sometimes 4 days! If they don't get along after a couple of weeks, you may want to try some calming helpers like Feliway spray and/or plug-in or Bach's Rescue Remedy, that will help smooth things out.



You got a good reply here and there is not much more to add, other than if after a couple of weeks of separation, things are not working out then talk to your vet about some sort of antianxiety meds for the cat. Usually these work and the cat unlearns the aggressive behavior. Then the meds can be gradually withdrawn.
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Smile, it will make them wonder what you are up to!

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