Husband has relationship with a relative lady who has 2 child

Sir i m living in Delhi, my sister is in Jhansi, her husband has relationship with a relative lady and he is beating , abusing and last night he try to kill, she is living with him after inlaw family force to live with him to solve problem. actualy he is in police and posted at Banda(UP). he has beaten very badly and she try to escape but Grand mother inlaw close the door intentionally. He call her Grand mother to start cooler and bring rope to hang her....she in deep depression and she told me dont do anything because he threatened that he will kill me and my brother sons who is the only boy in our family. She also try to go nearby police station but he got information and take back home forcefully. Sir my sister has one boy child and married in 2006. I am unable to understand what will i do......i told her to go police station we will come from the back but she stop us under threaten conditions. I have all my voice recording of my sister who has told me everything.....Pls suggest a best sections and way to fight under which sister husband will get life time jail. Regards ABCD