My 2 cats (brother and sister) aren't getting along anymore. Everytime Charlie goes near Amber she growls and hisses at him. I have had them for about 6 months and they are just over a year old. They were fine for the first 3 or 4 months, sleeping next to each other, play fighting, etc. But it seems to have started since I had a cat flap fitted, when Charlie had been out and returned, it's as if Amber didn't recognise him and thought he was an intruder cat. To start with, she eventually would come around, but over the last week or so she cannot stand to be around him - Charlie seems a little baffled by it all. Anyone else had a similar problem?