One guy in my family(extended) was fighting with her own blood sister(nuclear) and i went there as a brother in the same family to separate. With all my effort to stop these guys,the boy told me I should not involve myself in thing of nuclear family that if he kills her sister is never my headache. I thought he is saying the truth; but,if this continues,how will 'be your brother's keeper' exist or 'let brotherly love continue' be practised?

You should do whats right at all cost,,, if you leave him to kill her,, he will hate you when he comes to his normal sense.. So bewise