One of my cats has been very calm and easy going for the first two years of his life and now, all of a sudden, he has started to hiss at us. It seemed to start as a game he was playing. We would be walking and he would jump in front of us and hiss. Now he has started hissing all the time. When we pet him, pick him up and sometimes if we just look at him. He is also being aggressive with the other two cats, one of which is his sister. I am really not sure how to handle this anymore. Any suggestions?

Has he been neutered? I don't know a lot about cats, but I know male cats can start acting different if they are intact.

Is the cat neutered? Once cats hit puberty, hormones can drastically change their behaviour. They can start to see themselves as separate from the family or "pride", and will act out or challenge others they consider a threat to "their" territory. Sometimes it can manifest fairly quickly, but taking the hormones out of the equation doesn't always solve the problem, sometimes I think cats are just plain crazy. My suggestion would be to neuter him if he isn't already, give him plenty of space or at least his own little area, or film everything because most people think seeing others attacked by cats is hilarious and you could make a lot of money on a home video show. Good luck!