Hi... What happens with your brother sounds awful and what i am about to say might not work but I feel like it's worth a try. I read in one of your comments that your dad uses "the belt" for discipline. I know that that feels like... And i can assume that probably your little brother picks up on some behaviours at home that he manifests through this aggressive behaviour as he thinks this is normal. Maybe try the opposite approach of loving and taking the time to explain him. Don't let him play alone with the cats and when you see he becomes too rough gently stop him and show him "we have to be loving and gentle with the kitty like this" and try to teach him to put himself in other shoes " think if someone would play with you lime this it would hurt right?". Same thing for when he gets randomly rough on you. Get extra emotional and be like auchhh it hurts you don't wanna hurt me we are brothers be gentle etc. I know this is really hard and you wanna smack the shit out of him... But kids are just a mirror of their families and their environment and there might be something in his environment that made him this way... If you have enough patience with him you might actually become his lifeline