I adopted two kittens 3 years ago and they are suppose to be brother and sister. The are both fixed and have always got along pretty well for cats. After living together for 3 years the black cat the female started growling and hissing at the other cat. I put her in the bedroom so they both could calm down. I let her out after awhile and they were fine. The two days later it happened again and it has been going on for the last 4 days. I tried it separating them again, but everytime they see each other it starts all over again. I do not think that the issue is over territory because they will still share a litter box and take turns eating. I was hoping they would work it out and getting rid of one is out of the question because it is my responsibility to take care of them both. Do you have any suggestions?
cats behavior is always based around territory, food, mating and personalty. Other than that cats don't have issues like arguments like humans do. So although you say it can't be about territory, i wouldn't be surprised if this has become an issue for them. Why all of a sudden, well that we will probably never know. I get asked this sort of question all the time about why cats who normally get on suddenly don't.
I know you say that they both share a litter tray but cats don't actually like this much as the smell of the other cats urine is a marker of territory, so by one cat urinating over anothers it is sending a signal of challenging for the territory, maybe this is what is happening. I always suggest that cats even from the same litter should always have their own separate litter tray (not near each other) and feeding bowls etc. It allows each cat to have it;s own territory. This may help them to settle down again once they feel that they are not under threat by the other cat.