I have 2 cats, they are sisters so been together since they were kittens. They are about 2 and a half now. They have both been spayed. Anyway the one has started picking fights with the other over food, and hissing at her if she so much as walks past her, they do get along alot of the time. Just wondered if this is something to worry about, as she never did it before, and is/was the more laid back of the cats. She is great with the kids, dosen't like me much. Any ideas. Thanks