You have nothing to be guilty about as you didn't hurt him..your dad did. As far as not telling your mom, I suspect she already knew but for some reason dismissed it, maybe out of fear it would get worse?
All you can do is make sure you make your abused brother feel very loved and let him know you support him and feel bad for what he went through. Hopefully he is a strong young man that will one day let it pass and make a good life for himself. Obviously he will never forget, but knowing he is loved by you and your other brother should help him heal.
All my very best to all of you...JJ..