aggression against siblings

i have a female and male cat i rescued about 6 yrs all of a sudden my male is very aggressive against his sister he is beating her up for no reason. can somebody help me ? i have ferrel cats that live outside but they have been there for years all are fixed what is his problem? help!!

I think this is a case of redirected aggression. The aggressive must have seen an outside cat that got too close and since it wasn't able to get to the outside cat, it attacked his sibling. They need to be completely separated and reintroduced slowly, otherwise the fighting will escalate. You can read all about redirected aggression online. Good luck to you. Patience is key.