My family owns two adult female cats (Lily and Daisy). We got them at a Petsmart when they were kittens. They are both of the same litter and were in the same cage when we adopted them. They're both spayed, indoor cats. We've had them for almost nine years now and they seem to have increasingly higher hostility toward one another. They fight all the time. It usually starts when they're sleeping together. They groom each other and suddenly become violent, scratching and biting at each other. And then, for the rest of he day, whenever they're in the same room, they growl and hiss and, if they approach one another, they'llbegin to fight. Now they've both got scratches across their noses and I worry every time they're in the same room that they'll fight again.
normally when this type of behavior shows itself between two cats that have previously got on together it is associated with an event that one if the cats has experienced that has upset them and they have associated with the other cat. It is called Trauma aggression. Sometimes it can even be something that has nothing to do with the cat, ie a loud noise, tail being stepped on etc. but for some reason they put the blame on the other cat, this is common.
It may be that you will have to separate them for a while giving them time to get over the associated fear. I often recommend that you go through the steps for introducing news cats to each other as it helps to build the bonds again but in a slow and controlled environment. I have included two links to my pages about this at the bottom of this page. Hopefully they will provide more information on this subject.