wow. This subjct is overwhelming to say the least. As much as many of us are enraged and concerned but no-one knows this particular instance better than Ashi and it appears he is taking active steps to improve the situation.
Ashi, many here have given you excellent suggestions but ultimately you and your family have to decide how to handle this in the best way. Please be assured you have support here for your decisions.
(Added in edit) Should you want someone (a non-involved third party) to make the initial report to the authorities please feel free to contact me. ( [email protected] ) I have done this before for people to cut back on retaliatory action on immediate family members by the abuser)
How is the 14 year old boy doing? What are his feelings and thoughts? I am glad to hear you have opened your home before but he may feel guilty about deserting his mother (i.e. - running away) and may need a lot more encouragement to do so to help him appreciate that such an step would not be cowardly.
14 year old boys have weird perceptions of valor at times.
The man (I hesitate to use the term father as it would denigrate the many excellent 'real dads' out there) defintiely needs to be called for his actions. Please DO NOT take revenge matters into your own hands. This will only give him ammunition to hurt you and your family more. Reporting is a must tho but even before such happens get the boy out of that situation.
Who knows what is going through your brother's mind right now? Running away? Thoughts of murderous revenge? suicide? Acceptance that his role in life is to be a punching bag? Getting engrained that beating people, i.e. his future wife and kids, is okay?
Who knows the thoughts that go through the mind of a 14 year old boy who has been physically abused. He needs your love but more than anything right now he needs your protection, then your love. Don't force him to do it himself or worse succumb to the mental conditioning that has already been going on for 14 years.
Just my non-professional two cents
Kismet