My cats are brother and sister from the same litter. We got them when they were 8 weeks old and moved to our new house when they were 10 weeks old. They were fine and settled and we let them out but brought them in at night. Getting on very well, playing together and cleaning each other. Then we fitted a cat flap at the end of March and since then one really hates the other, they attack each other if they are in the same room. Both have got very much closer to me and one is quite clingy now. This has been going on for 2 months now. I've heard that maybe one is coming back in with new smells on it from outside and the other is disliking this. I wondered if its a jelousy thing becasue i'm closer to them both now. We live next door to 4 other cats and across the road form 2 others, some get along and some don't. Any ideas what it is or how long it will last??? They will be 1 year old soon and both were neutered at an early age.