Sir as far as your sister is concerned she can file an domestic violence and 498a along with dowry case on in laws. She can file a complaint against the father in law and mother in law. Stating the complete scenario and there acts of harresment in the complaint against her and her husband.
Further see as your brother law has just recieved salary and he is not having anything on his name he shall be treated as employee and cannot claim his right on business in name of father. So it better he should start doing something of his own.
Further for your parents to file complaint a police complaint can be made for harresmnent and intimidation for murder agaiant the father in law.
Further in my view it would be in best interest if for time being they both stay separate do their work separately as brother in law can loose inheritance going against them at this stage as may be in my his sister is interfering for the property.