You should NC to the police in written intimating them that your sister in law is threatening your father and your brother file false case against them in future just to assert her dominance.

Please remember to take due knowledgement on a copy of the complaint given to the police. This can be used later in court if your sister-in-law goes on to file false cases against you and your family.

1) Ask your father and brother to keep evidence or proof whatever she says like false threatening and abuse words, same produce in the police station as no complaint base. So police can have idea about your sister in laws behavior.

If she is preparing, what is your brother doing. 

Solve the misunderstanding instead of simply thinking that she is going to file some case.

Let me assure you, courts cannot give you any remedy in matrimonial matters.

They will simply pass some order which is not beneficial either to petitioner or the defendant.

Solve the matter amicably before going to court. 

One thing is for sure, if she files some case, the case will run for 1–2 decades, everybody including her will suffer. Whether you are guilty or not that will be proved by the end of the case. If proved guilty, who all are named under 498a, all will go to jail for 7 years.

If it is proved that she has filed a false case, then you will have to file separate case asking court to send her to jail for lying under oath and wasting court time.

All these will take 1 to 2 decades to get over. After going through all this, who will stay together? They will want to file divorce, that will take almost similar length of time to get over, if parties dont mutually consent for divorce.

So instead of wasting 1–3 decades in courts, simply pay what the girl wants or simply adjust with the way she wants and you can either continue leading happy married life with her or simply take mutual divorce and move on.

Your father must immediately lodge a Police Complaint, detailing therein all the incidences of violence and harassment that is being meeted out to him. 

She can file false dowry, 498a and domestic violence case alongwith maintenance to harass all of you, if your father can file domestic violence case against brothers wife, it will work wonders, i can guide you further in detail

Record the threats 

2) file police complaint against sister in law and her parents for criminal intimidation under section 506 of IPC 

Hello,

You can not do much in anticipation. 

However, if false threats are being passed then you may go ahead and file a NC in the police station, so that if a case is filed later, you have some defense to take. 

Regards 

Sir see firstly if the relationship with brother and his wife is  not good it would be wise for sometime to move father to your place as if she goes for allegation on your family the false one you can take defence that you lived seperately but the father is in share household so that can have some impact it shall be precautionary measure.

Secondly you along father can approach police and father NC agasint the wife and family for intimidating for false case and making whole family suffer. it would be better if proof of such false intimidation are recorded and gathered to protect interest in future.

In order to safe guard your slf and your wife atleast you may file a NC

You can file NC with local police station if you so desire 

Approaching police over complaint will not be a good idea right now, thorough discussion of pros and cons need to be understood

NC will go a long way in your case. Also,  a complaint can be made by your father under the provisions of Maintenance and Welfare of Parents and Senior Citizen's Act, 2007 against your sister in law for harassment meted towards your father. 

He should also resist and speak where his wife goes wrong.

Make a Police complaint at the earliest. 

Sir  it would be better to take precautions and try to resolve issues with her so that he can lead to smooth life. 

1. Well, irrespective of any number of complaints you lodge with local police you can not stop your SIL from filing a case even if false u/s 498A IPC.

2. Hoever there is no nothing to worry about such case as even if such FIR is registered you can easily get bail.

3. Under the present circumstances do not let your brother or father to stay with your SIL anymore and ask your brother to amicably settle the dispute.

1. Collect evidence of her and her family members' giving such threats of lodging false complaint against all of you alleging torture on her.

2. This will be your shield in case she lodges any such false complaint in future.

3./ Make sure that you do not keep any contact with your said sister in law.

1. If possible, ask your brother to secretely instal  a CCTV or spy camera in her home to record  the said conversatiions of his wife giving hraet o0f lodging false police complaint again st you.

2. Such recording will be considered as very valuable evidence later on.

if till now they have not filed any complaint against you. Then your father should disowned them and he can moved an application of police that his daughter in law is harrasing him and threatning to implicate in false case.

Why they are threatening your brother and father without any reason, ask your sister in law if she has any problem come to know the actual issue 

You may file NC in your nearest police station

Yeah,  You can protest you brother fomr doing all these things as well.