<human>: My girlfriend and her mum both get beaten by her brother. Since we have been going out (4 months) he has only hit her once (it was a weak punch so i dismissed it) but he repeatedly punches and kicks his mum to the point she is crying. He hasn't hurt my girlfriend since we have been dating but before that he has given her bruises across her body and a black eye. I am not entirely sure what to make of this situation. I told my girlfriend that if he ever hits her again to tell me but im not entirely sure what i should do if he did hit her again. He is by no means strong so i have no problem with having to beat his ass but he is still her brother and she will be upset if i was to hit him. Plus, ill probably have a hard time coming back to their house because i have no idea how his mum will react to me beating up her son. She has never called the police and i doubt she ever will so im pretty sure if i was to hurt him i would not be allowed back in. The husband/ father passed away a couple years ago so that has caused trauma to the family but also there are no rules restraining her brother. He gets what he wants and if his mum refuses to get him what he wants he beats her until she does. What should i do? the situation with his mum is not necessarily my business but what should i do if he hits my girlfriend again?

Note: also i very much love my girlfriend and cant stand the thought of her being hurt.

Can you condense the text into three sentences or less?
<bot>: my girlfriends brother has beat her numerous times before we dated, should i beat his ass if he does it again or if not what else can i do?