You can file a police complaint immediately and seek police help

You can also file the complaint without father's support

How do you know that girl told your father to murder you and your mother ? 

Install CCTV cameras in your house 

3) file police complaint against your father and the girl 

File police complaint against your father 

Your father is bound to maintain you and your mother 

if he refuses your mother can file  DV case against her husband seek protection order, maintenance, compensation for mental torture undergone by her , injunction restraining sale of matrimonial house  by her husband 

First get hold of some proof or video

contents of your query are not clear,

however, your mother can file a case of divorce on the ground of adultery (having a relationship with the said girl) against your father and can claim maintenance, alimony, etc

She can also lodge Domestic Violence complaint 

Hi,,

You are suggested to file complaint regarding threat to life against your father and that unknown girl.

If she ever visited your house then move a complaint against her that she is threatening to face dire consequences and further she is threatening and demanding Rs. _ lakh or she will file a case against you. 

Police will register a case against her under section 506;384

Or your mother can move an application on behalf of your mother that she has trespassed in your house and trying to rob her

Hi

First find  out the  whereabouts  of  the unknown girl.

Options:

1) Follow your father.

2) Take  photos 

3) collect the phone details of i.e., calls, WhatsApp of your father.

4) If your claim is correct, after above, file complaint  with local police.

Good Luck

Please file a police complaint in this regard against your father and unknown girl if you have any evidence in this regard you must provide that evidence   to police to get the matter investigated

See if father threatens and intimidate you and your mother you can file complaint of same. Also your mother can file case of domestic violence and cruelty against father can seek protection , residence and maintenance from him form court.

Filing case agaisnt the girl is not possible in absence of evidence agaisnt her.

Claim maintenance from the father, mother can file matrimonial case agaisnt him.

Dear,

       How do you know.

       You know that girl.

       Collect some proof and file police complaint against him through your mother name.

Dear Sir,

yes, you can lodge a complain. You go to the nearest police station and lodge a complain against that girl and your father.

Ask your mother to file FIR for threat to her life from your father.

your mother can file criminal complaint against her. 

your mother can also file Domestic violence against your father. 

your mother can approach before jurisdictional women cell. 

Dear Client,

Firstly, you have to inquire about the unknown girl and what relation with your father.  

Secondly, you and your mother have to complain/lodge FIR before police station.

You may call me through KAANOON for more detail.

Do you have evidence for that i.e., of she having told your father to kill your mother, if so your mother can lodge a criminal complaint against that lady for the persisting threats from that lady in this regard.

Without any evidence you may not be able to take any action against that lady.

How can your father support your mother if he is under the control of that lady.

Your mother has to adopt some strategy to drive that lady from your father's life.

You may consult an advocate in the local and take his advise on all such issues.