The general consensus question is are they neutered? I am gonna take a stab in the dark and say yes. Second question do they go outside at all? Years ago I had littermates, male and female. Always a peaceable kingdom. They only went outside under my strict supervision. Male got some strange smell on him. Female attacked and I mean attacked him big time. Poor guy was all sorts of confused about the attacks. I bathed male, attacks persisted. I wrapped male in towel, then wrapped female in same towel to comingle scents, still attacks and I mean vicous ones. After about 4 days I cart the female off to the vet and he said a smell can set them off like that and even though I eliminated the smell the behavior was still persisting. The only thing we could do to break the cycle was medicate the female. I called them kitty qualudes. Worked like a charm. She only had to be on them for about a week or so.
My current crew consists of 4 neutered males and 1 spayed female. All get along. But the occasional play does end up turning into a bit of a fight but usually the agressee runs away from the agressor. Rarely does fur fly. All of mine co-habitate so well I can not tell who is alpha in the bunch.
Like many others said they could be trying to establish who is alpha but if the problem persists and escalates a call to the vet may be in order for a scrip of kitty qualudes :lol: