I have two cats who are 9 - not litter mates but had them both from kittens and they've always got along fine for the most part. We've just moved to a new house (3 days ago) and one of them is now sniffing at and then hissing, growling and swiping at the other. Another cat used to live here so I expect she can smell it and that's part of the reason why. She also does this any time they come back after going to the vet but it always stops in a few hours. But it seems to be going on longer this time.
Just wondered if anyone else had this issue and if so how you resolved it!