Hello,

lodge a complaint in the police station for criminal intimidation. Approach the police at the earliest.

regards

sister should file police complaint against her husband for criminal intimidation under section 506 of IPC 

2) she should record his threats . enclose audio recordings transcript in her complaint 

She can file a police complaint immediately and inform the police. Police may provide her security for the same

Your sister can file a complaint with the police for the offence of criminal intimidation under section 506 of the Indian Penal Code.

Also, she should collect documentary evidence to prove her claims in the form of audio or video recording.

Through what mode he is threatening her ? Message / calls / in person ? Record the threat and file criminal complaint of Criminal intimidation to death. 

Complain to police. This is only way against any threat. 

Respected sir... 

There are two things one is that in which you have to convince her to be normal and second to take legal action against him... But there is loop hole that your relationship will be distributed... Make your mind clear and then take action in legal action you must have to lodge FIR against him... 

Thank you

File a police complaint against the brother in law for intimidation seek police protection for sister as there is threat to her life.

your sister should lodge criminal complaint against him under 506 of ipc and better try to record audio calls. 

Dear Sir,

File a case u/s 506 for threatening of your sister in Police station against your brother in law.

IPC

Chapter XXII

S. 506

Punishment for criminal intimidation

Description

Whoever commits the offence of criminal intimidation shall be punished with imprisonment of either description for a term which may extend to two years, or with fine, or with both;

If threat be to cause death or grievous hurt, etc — and if the threat be to cause death or grievous hurt, or to cause the destruction of any property by fire, or to cause an offence punishable with death or imprisonment for life, of with imprisonment for a term which may extend to seven years, or to impute unchastity to a woman, shall be punished with imprisonment of either description for a term which may extend to seven years, or with fine, or with both.

If he is giving open threats this way, your sister can lodge a criminal complaint agaisnt him with the police about this and get a case registered against him while seeking protection.

Your sister should record the calls and lodge an FIR for criminal intimidation against her husband immediately. 

Ask your sister to break all contacts with him and don't let him know about her whereabouts.