Domestic Violance on my elder sister

My sister got married in 2006 and have one son.In the begining relationship between my sister and brother in law was good.But since last 2 years she is complaining of physical and mental torture by brother in law and his family members.She complained me that brother in law came home everyday night drunk and torture her.Brother in law family member also support the act she complained. Brother in law father is a retired govt servant &mother is housewife.Brother in law have a eletric shop which is non-operational and barely get any income.Please guide me what should be done.