If you dismiss him from the job he won't be able to maintain your sister.It is better soicailly solve this.
If he is cruel enough then let your sister to be away from him for some time, she can get maintenance from from your BIL and he is bound to miantain her. Let her live away from him for sometime till good sence prevails upon him. You should try to look out the casue for him for doing this as to protect the peaceful life of your sister it is must to know the root cause.The remedy in law is alwys there but it is much preferable to solve such soical issues socailly by involving the society in it.
If you or your sister try to meet or inform the boss of your BIL may be he will harm her more . If the situation is endless the you must land such matter in the office of your BIL before his boss.