Why is my cat hissing at his sister after she was outside for weeks?

Thank you for submitting your question regarding your cats. I suspect that Stella is stressed by the return of her housemate. Callie likely smells and acts a bit different after being gone for so long. I would keep them separate to allow them time to readjust to each other. They should get along fine, but it will take some time. I hope this information helps!

Cats exhibit this behavior sometimes when one leaves the house. Most of the time it’s more growling and hissing and running then anything else. Most cats will get over this quickly. Supervise them so that they don’t hurt each other and make sure one does not bully the other. If you have something they really like it such as canned cat food, you could try feeding both cats the food or treats close to each other and talking sweetly and petting both. I hope this helps and best of luck!

It's possible the stress of your being gone caused the behavior change. I would give Hippy a few days to rest, relax, and make sure she can get to her food, litter, places to hide, etc without being bothered and see if that helps. Sometimes, pain or underlying illness can also cause a cat to suddenly lash out and hiss or swat at other cats, so if the behavior continues or you see visible signs of illness, a vet visit to rule out health issues is best.

Did Pernell go to the vet recently or leave the house for a period of time? When cats go to the vet or leave the house for a period of time (such as going to a kennel), they can come back smelling differently and that would cause the existing cat to view the returning cat as a stranger. Then that would lead to aggression. Or it could be that Gattino isn't feeling well. If he was in pain that would causing him to react aggressively to Pernell. I'd recommend a check up with the vet for Pernell to make sure everything is ok. I hope this helps!

If your cats have spent most of their time outdoors and now are spending time inside together then this change in environment could be causing tensions. If they are still outside all the time then it is possible that a new cat has come into the neighborhood and this has disrupted the social structure of the cats, or potentially one of the cats could be feeling unwell and therefore being more defensive, or they could feel they are having to compete for food/shelter. I would be sure both cats have equal access to food and water (they may need their own separate areas to eat/drink so they don't feel threatened) and litter boxes indoors. When indoors I would recommend the use of the Feliway Multicat diffuser https://www.amazon.com/CEVA-Animal-Health-D89410B-MultiCat/dp/B01AVHPFDS/ref=sr_1_4?ie=UTF8&qid=1503838561&sr=8-4&keywords=feliway+multicat, which is a pheremone product. I would consider an examination by your vet to look for underlying health problems as well.

If only pets could talk! Cats can be very reactive and can also get frustrated easily. It’s possible they saw another cat out the window and are reacting. You may try to separate them for a few hours. Keep the environment calm and check on them frequently to make sure they are not agitated. You want want to close the blinds to block out any views of the other cat. If the behavior continues, have them examined by the veterinarian. I hope this helps and please feel free to post any additional questions.

Poor Atta! Abnormal cat behavior can occur for many reasons, and often it is difficult to figure out. If her sister was out of the house she may have a weird smell to her, or if something startled Atta she may have projected that onto her sister. Whatever the reason, I would leave her alone for now. Put a litter box in the room where she is hiding along with food and water and just leave her be. If she is still acting off tomorrow, I would have her seen by her veterinarian as sometimes this can be caused by pain or a medical condition.