I have 2 female cats they are 6 years old and have been together from birth. One of them has started hissing at the other one every time she comes in the room and she tries to go for her if the aggresive one is on my knee I push her down if I am around I shout at her but just womndered if anyone can explain why this may be happening, its only started over the last couple of months.
Perhaps sb if you contact the vet he may know of a cat behaviourist he could put you in touch with & for a fee probably they could help you.
Just a thought. Hope you solve the prob soon. Let us at AB know.