If it is your mother's own money then she cannot be prevented or restricted to transfer her money to anyone including to your sister. It is not an illegal act. If you feel that your mother is mentally insane then the burden lies on you to prove that your mother is suffering from mental illness before court and apply court guardian before the district collector within the district she currently resides. You cannot file any case in Delhi if shew is residing at West Bengal. 

You cannot restrict your mother from sending her own money to your sister or anyone, including spending the money at here will and wish 

your mother being owner of her account and money has every right to give her money to any person of her choice and for transfer or giving her money your mother does not require your permission nor require your consent. Even being you donot have any right. In capacity of nominee you can only receive money but is legally bound to distribute the same among class-1 legal heirs of mother. You donot have ant legal right to object. Your sister too is entitled to share in mother's property and money. 

A medical certificate is required to state that the person has a mental disorder that warrants psychiatric detention. The certificate must also state that the detention is necessary for the person's health and safety, or for the protection of others. 

As per section 329 Cr.P.C. :  

• If at the trial of any person before a Magistrate or Court of Session, it appears to the Magistrate or Court that such person is of unsound mind and consequently incapable of making his defence, the Magistrate or Court shall, in the first instance, try the fact of such unsoundness and incapacity, and if the Magistrate or Court, after considering such medical and other evidence as may be produced before him or it, is satisfied of the fact, he or it shall record a finding to that effect and shall postpone further proceedings in the case.
  
  1A. If during trial, the Magistrate or Court of Sessions finds the accused to be of unsound mind, he or it shall refer such person to a psychiatrist or clinical psychologist for care and treatment, and the psychiatrist or clinical psychologist, as the case may be shall report to the Magistrate or Court whether the accused is suffering from unsoundness of mind:
  
  Provided that if the accused is aggrieved by the information given by the psychiatric or clinical psychologist, as the case may be, to the Magistrate, he may prefer an appeal before the Medical Board which shall consist of -

  1. head of psychiatry unit in the nearest government hospital; and
  2. a faculty member in psychiatry in the nearest medical college;

• If such Magistrate or Court is informed that the person referred to in sub-section (1A) is a person of unsound mind, the Magistrate or Court shall further determine whether unsoundness of mind renders the accused incapable of entering defence and if the accused is found so incapable, the Magistrate or Court shall record a finding to that effect and shall examine the record of evidence produced by the prosecution and after hearing the advocate of the accused but without questioning the accused, if the Magistrate or Court finds that no prima facie case is made out against the accused, he or it shall, instead of postponing the trial, discharge the accused and deal with him in the manner provided under

Your mother should be examined by psychiatrist 

2he would conduct medical tests to confirm whether mother is mentally it or not 

3) after receipt of report decide your next step 

4) please note that nominee is only trustee for legal heirs and on mother Denise sister will have equal share in

money lying in bank accounts 

Get medical certificate first 

mother can during her lifetime transfer funds to your sister if she so desires 

Consult a psychiatrist 

Here are simplified steps you can consider:

1. Consult a Psychiatrist: Obtain a medical evaluation of your mother's mental capacity. This is crucial if you believe she may be mentally unfit to manage her finances.

2. Gather Evidence: If you plan to prove undue influence by your aunt or sister, collect evidence of suspicious transactions or undue pressure on your mother.

3. Obtain Legal Advice: Speak with a lawyer regarding your rights as a nominee, and the feasibility of challenging these transactions.

4. Understand Nominee Rights: As a nominee, you are merely a custodian of funds for legal heirs after your mother’s passing, and your sister has equal rights to inheritance.

5. File in West Bengal: Any legal case related to your mother’s funds should be filed in the jurisdiction where she resides (West Bengal), not Delhi.

These steps should help you assess whether pursuing legal action is viable based on your mother’s financial and mental status.

The grounds for taking this case to the jurisdiction of Delhi Courts:

1. Either of you should be a resident of delhi
2. Either of you must be carrying on any business for gain in Delhi
3. Any cause of action should have happened in delhi.

Ways you can prove her mentally unstable:

• You must have some psychiatrist evaluation of your mother and a history of mental instability from the past.

However, instead of finding a lawyer who can generate some grounds to prove your mother mentally unstable, you should be rather focussed upon finding a lawyer, who can get you relief of factual and legal grounds.

Good luck though!

You will need a doctors certificate in court for doing so. Otherwise you need to file a suit seeking medical examinaton of your mother

- Since, she is 78 years old , hence you can take the ground of influence of aunt and sister for restraining the bank to transfer the amount to them. 

- You can move an application before the Bank to stop the payment without your consent. 

- Further, you can send a legal notice to the bank being a nominee of the account.