I get the feeling that your father may have never wanted your middle-brother. That is, your middle-brother may have been an "accident". And as such, everytime the father sees your brother, he may get angry at what happened and what he is now responsible for.
THIS sounds like the main problem, and what fueled your father's anger towards your brother. Perhaps this anger has lessened over the years, but now your brother may have become affected by all that treatment, and developed anger himself. Your brother may now be expressing his anger to empower himself for the unfair treatment he believes he has experienced. And your dad, being the angry person he sounds like, may take your brother's anger as an insult to him, and respond with more anger. Its a sad situation.
Counseling may be the best idea at this point. Anger can lead to nasty things occuring, so it would be best to address this as soon as possible. If you can, try to give your brother as much sisterly love as possible, so that at least he is getting it from somewhere. Perhaps you can give him the love he feels he may not have gotten from his father.
Probably, you should notify the school psychologist or something like that. Or maybe your country has a telephone hot line for problems like yours? Anyway, you should not go through this alone! You need some adults and authorities to help you. Try to get help from anywhere possible, even the police maybe.
I think this isn't getting commented on, because the problem is really real and makes so sad. We don't know what to say.
I wish I could help you.
Your brother has been both emotionally and physically abused by your father, and he needs two things: 1. To get away from your father. 2. Professional help by a therapist, or maybe even better, by a psychiatrist.
Unless that happens, I'm afraid your brother will become just as your father is...