It is a problem I had for 14 years. Litter Mates born just 20 mins apart, with me acting as midwife, the 2 boys were happy as kittens until aged just over a year, when I heard a horrendous fight up on my roof terrace in the small hours. By the time I'd put something on and gone to investigate, there were no signs of a foreign cat so I assumed it had scarpered.
A few days later I heard it again but this time in daylight and I saw it was the 2 brothers. For the next 12 years or so I had numerous vet visits and a couple of Urgencia visits for me when I tried to stop them trying to kill each other.(in the end I got a large board and forced it between them). Not content with attacking each other they turned on my Spanish siamese, so it was like living in a war zone.
I tried all sorts of things from burning calming essential oils i n the house to plug-ins of Feliway and then Feliway Friends. Didn't make any difference. As they grew older they did calm down a bit and tried to avoid each other. They both died this year, one in mid january and the other at the beginning of february.
They had been neutered, so should have been much calmer.
